Per Rock Band's Twitter on how skill rating works:
We aren't detailing the literal ways, just aim to be a good player. ; )
And according to HMXdanbruno on Reddit
I personally calculate everyone's skill rating by hand. I wish you would all stop playing so many songs; it's hard to get anything else done now!
If you played this game before the Rivals update you may already have achieved this rating. Check out your "My Stats" under career and see if you have a number over 900. If you do, play a song or two on that instrument and it should unlock. To be safe, play one you already have Expert Gold Stars on.If you start from scratch on an instrument, it is possible to achieve a 1000 rating in as little as 7 songs. There is a "5" point difficulty scale which is actually 7 points. 0 dots, 1 dot, 2 dots.. etc... 5 dots and Devil Dots. Playing well enough on 7 songs to get Gold Stars, one from each dot rating on that scale, should net you a 1000 rating.
If you haven't played a certain instrument, you can start from scratch by playing that instrument instead of deleting everything.
All that matters to the rating is GOLD STARS and DIFFICULTY. If you have gold stars on all songs you've played (as little as one, as many as your whole library), all belonging to one song difficulty tier (the number of difficulty dots, the named difficulties are listed below next to their respective dots), and all played on Expert, your score will earn the 143 maximum skill score points. For each difficulty dot, all songs at that difficulty will earn 143. Total: 1001 (the extra 1 is due to rounding. Don't be surprised if you see 142 added for any particular group due to rounding.)
It might even be possible to get this achievement, if you start from scratch, with 0 effort. You'll need the songs below or find alternatives to them. Talkie songs you'll need to just make noise in the mic during the song for a perfect score. The youtube videos you'll need to sync the video up with the game and play it though the mic. I'm still looking for a solution for the two hardest difficulties. At this point, you should be able to get 900 with only TWO STARS on a Nightmare and Impossible song. You'd literally be right at the cut off, so in reality, aim for 3 stars on the two hardest difficulties that you can't use the talkie or YouTube trick on.
All talkie - Soul Remnants - Dead Black (Heart of Ice) [ON DISC]
All talkie - Rage Against the Machine - Bombtrack [DLC]
All talkie - Rage Against the Machine - Bulls on Parade [DLC]
All talkie - Pixies - Tame [DLC]
YouTube - Green Day - Homecoming [DLC] (use YouTube video below)
##### Impossible (in red, a devil)
-If you have multiple mics, you can have multiple people sing the same track and one one person has to hit the notes for it to pass.
-Don't play a song if you know you can't gold star it on expert.
-If you've already played a song, but can't gold star it, you'll have regular 5 stars.
-Once you've raised all your played scores as high as you can, you can begin playing other songs, only playing those that you can Gold Star. I assume it averages out scores, so the more songs you have that you can't gold star, the more songs you'll have to add that you can gold star to overcome the averaging.
Check out this extensive Reddit thread as well: https://www.reddit.com/r/Rockband/comments/5a3vqx/instrument...
(I've removed references to Full Combos based on this Reddit thread)
Also read the comments on this thread, some people have added some insight into how things might happen in different situations. I've also combined many ideas I've read in different places including Reddit and the comments below.