Not sure if this has been posted, but I just wanted to put this here just in case anyone needed help obtaining 50 or a 90 star skill rating.
I personally used these time trial tutorials done by ovendonkey186 on youtube. He has done a tutorial for every time trial
in the game (YES, even the pure time trials), his videos are long and detailed. They all follow the same format:
He goes through the stage commentating on what the player should do and tells them the fastest way to get through the stage
Outlines any hard jumps and shows the viewer how to do them
Shows you a real 3-star run from start to finish
He used to be a world record holder for many levels when it came to time trials (although they've been beaten now) so he knows what he's talking about.
This is the playlist:
Also, he uses the "side jump boost", but that isn't necessary to learn.
With this guy I earned my 90 stars (from 50 stars) in less than a day.
Also, he does tutorials for the speedrun achievements which is a bonus.
If you want a taste for what his videos are like, here is one of his tutorials: