I’ve done it for my son’s Joy Cons and it fixed the drift completely, as if they were brand new.
****
***Disclaimer: Do at your own risk. You need some skills and tools to do this, you will void any warranty, and you may permanently damage the Joy Cons if you are not careful.***
****
The short: Open the Joy Cons, pull out the battery (but not the cables), remove the battery holder (but not any cables), put a piece of ~1mm thick cardboard on the back of the analog stick’s housing (same size or slightly smaller than the metal backing of the stick), reassemble everything. Enjoy drift-free Joy Cons.
Reasoning behind the fix: If you have drift, you can push/press the housing of the Joy Con around the stick with your fingers (see the video below), and the drift will vanish. Adding pressure to the stick housing may thusly fix the drift. Experiment shows this to be true.
(Not my videos, no affiliation, just give the guys some thumbs ups if you like it)
[How To Fix Your Joycon Drift Permanently](https://www.youtube.com/watch?v=Vid8lIXmZwE) important part from 5:55 on.
[How to tear down the Joy Cons](https://www.youtube.com/watch?v=m63sRY_1eOw)
(Video shows one side. Other side is similar but has different screw placement for the battery holder.)
It took me about 15 minutes to do this for my son’s Joy Cons and they work as if factory new now. Their drift made them unusable before.