A Brief Introduction
Learning decent technique on guitar is like wading through a big swamp of collective shit
I remember about two years ago, when I was looking up some information on thumbpicks and their proper technique in general, I couldn't find anything truly relevant. I was interested in learning to use one at the time but the lack of technical information left me rather apprehensive. The articles I found ranged from uncited accounts regarding tendon damage to a sunny outlook of "Keep doing it you'll get better in no time!"
I'll do my best to try and explain the exact differences in using a thumbpick compared to the bare thumb in physical technique and the possible dangers of using bad form. I'll also give a more detailed guide on how to make sure your fingerpicks fit you correctly with an emphasis on alaska piks; due to the increased variables involved in their adjustment.
Thumbpicks
Polyoxymethylene!@!#!#
Most picks you're going to see in a music store are made of "Delrin," a trade name given to a thermoplastic called blahblahblah (it's in the sub header) that's stiff and durable. They're also sometimes made of polycarbonate AKA riot shields fuck year; the material tends to offer more durability than anything else and has a tendency to scrape on the strings giving an apparent "picked" sound.
The obvious stiffness that these materials offer are very much noticed when you try on a thumbpick for the first time. Chances are if you're trying on generic ones you won't find something that fits comfortably. This is already essentially half the battle, finding the right size that fits your thumb. If it's too tight, you'll quickly become uncomfortable while playing for the shortest of periods. Too loose and it'll slip out of position forcing you to stop playing all the time. The majority of these picks are simply casted from general molds. Don't hesitate filing down edges or the tip to better fit your preferences and comfort. You want to achieve the greatest degree of control over the thumbpick, the better it fits you, the easier this will be.
First thing's first though, when you wield a thumbpick it should be in the same position you'd wield an average pick; the thumb itself being almost parallel to the strings. This is unlike using your bare thumb, which involves more of a 20-35 degree angle of attack to the strings. If you're just beginning in learning to use one I highly suggest you wield it exactly like a regular pick, pressed against the side of your index finger.
It shouldn't be a priority to instantly go into playing normal fingerstyle as you would likely want to, but to first familiarize your muscles to using the pick. Practice basic alternate picking, if possible, as well as the basic down strokes. This process doesn't take long at all if you've some previous picking experience of some sort, but even if not, it greatly speeds up the learning process. The introduction of the other fingers means you will no longer be able to use your index finger for support a la a regular pick, therefore, try to be as gradual and patient as possible. Don't jump into fast picking and get frustrated if you find you can't pick the bass strings as fast as you can with a bare thumb. Try to take to mind that even though you may possess good fingerstyle technique it doesn't automatically make you able to pick up a thumbpick and play exceptionally.
Remember! Just like with a regular pick, the majority of the power should originate from the wrist, the rest should be supplied by your base thumb joint while the thumb knuckle joint serves as a pilot and director to where you're picking. If you find yourself becoming sore quickly, analyze your playing carefully and correct accordingly. This method is doubly important for mastering thumbpick upstrokes.
Choosing the grade of thickness of a pick should be dependent on how you want to play. Thin thumbpicks will require less torque for faster down strokes while thick thumbpicks will remain firm and steady for pulling off upstrokes. If you find yourself barely adjusting to picking technique then I suggest starting with a Thin grade and working your way up over time.
Here's some thumb pick variants to check out:
Fred Kelly Speed Pick

Fred Kelly Jazzy Flatpick Hybrid Thing

Whatever The Fuck This Is

Fingerpicks
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One of my constant troubles with playing fingerstyle on a steel string was being limited by the strength and durability of my fingernails. Slowly over time I began to become gradually more frustrated with not being able to play with the power I wanted to let alone the trouble with consistent harmonics at the 5th fret. Growing nails was one of the best things I did for my growth on guitar, but over time it certainly became apparent to me how much more they were meant for nylon and not steel.
An old secret used by last generation steel string players like the late Michael Hedges was to glue slices of ping pong ball underneath the nail to give a louder more consistent sound. This added a little more durability to the nail but there aren't very many players, including myself, who'd venture to glue something under their nail. At the other end of the spectrum was the fingerpick. Used more for folk at the time, many have changed very little. Still mostly made of nickel, brass, or polywhatever, they enable a more consistent sound without the worries of breaking a nail. The cost usually being in no longer being able to quite play the same way; most traditional fingerpicks are essentially like thumbpicks, finger flesh not usually being part of the equation.
Newer fingerpicks put more emphasis on trying to grip to the nail more to give a more familiar feel. Though I think on of more popular designs seen among upcoming fingerstyle players like Antione Dufour, Ewan Dobson, or Sungha Jung is the use of fingerpicks called Alaska Piks. These are essentially a combination of, in my opinion, the best of both worlds when it comes to playability and durability. They utilize "Under the nail over the finger" construction that has a very similar look to, say, placing a piece of ping pong ball underneath the nail, whilst having a secure hold on the finger granting much personally longed for durability. There's hardly a learning curve compared to a thumbpick;if you can play, you won't have much trouble getting used to them. This is what I currently use and suggest everyone give them a try. Though there is a little finesse when it comes to setting them up and making them comfortable for your fingers.
Setting up your Alaska Piks
Directions included my ass
I couldn't find a single place on the internet that tried to help clarify and explain how to set up alaska piks, so this will be the little niche part of the article for the sake of explaining how.
The website here informs that directions come with the order but they're essentially a tiny blue paper detailing the same information on the website as quoted below:

Custom Fitting Instructions
1) aLaska Pik's TM plastic and brass picks can be manicured to precise length: (Plastic) use clippers, then file. (Brass) use file or grinder. To manicure, place pick securely "Over the finger, Under the nail," decide the length desired, then clip and file. For a side of the fingernail attack remove plastic at indicator #1.
2) To match the curvature of the user's flesh under the nail area, file on the backside of picking surface (indicator #2). This adds security. This should be done before changing length. 3) To un-stiffen pick remove all or part of the pressure band (indicator #3). For added comfort file smooth all uncomfortable edges. The brass pressure band should be bent inward. This creates tension on top of the fingernail (indicator #3).
Now that shit is a bit vague so let me do my best to explain the best way to go about it. First thing first, be sure to file any sharp edges. These things are basically cut out of molds and none are truly designed to fit perfectly without a bit of tweaking. These first few steps are solely to make it more comfortable for your finger.
Next, if it feels tight, which it probably will, don't hesitate to cut the middle of the pressure band at #3 in the picture. It'll still be firm and secure, there's no need to have it uncomfortably clamped on your finger. You can make it loser by cutting a larger segment out of the middle of the pressure band. Now chances are it'll still clamp a bit tight around the finger, feel free to expand it open to make it fit more loose. Now move on to the exhibit #2. There's a bit of an edge where your nail slides over, I suggest filing it with some sand paper so it's smooth and doesn't irritate the bottom of your nail over time.
Now a bit of an interesting part. At #1 it says to remove a piece of plastic depending on side of fingernail attack. Basically, it'll usually be the left side as shown in the picture that is snipped out if you're right handed. Grab a pencil and while wearing the pick place the according finger on string as if you were about to pluck it. Mark where the plastic touches the string with pencil with a simple line. Now take your nail clippers and press the left side of the clippers along the line to about the midpoint. Clip and it should, more or less, give you a nice right angle. File it to smooth it out and move on.
Lastly, comes the actual pick shaping. I don't suggest using clippers like the directions say, just file it down while wearing it like you would your normal nail. This part is all preference, file it to how you keep your nails. Buff it, and bam, you done punk.