Funny, I'm a Sheik main who used to second Ganon, but then I played around with Pikachu and now Pikachu is my 2nd.
I actually think that the 2 characters are somewhat similar in a couple of way. Both have fairs as big weapons so you spend most of your time facing your opponent, but your mobility is not quite good enough to always approach with a SHFFL'ed fair. You can sometimes, but it become predictable and punishable. Sheik's is definitely hard to punish, but still similar.
I would say to start, you can think of Sheik as a faster and smaller... and more boring depending on who you are... Ganon without the WL gimmicks. Oh, and you have a projectile now too. ^_^
I think when picking up a new character, you should start with the fundamentals. To me, those are spacing, movement, and punishes.
Spacing- know the range of you moves. Not just where the hitboxes extend to, but things like, how far will my dash attack hit, or how far will my WD OoS reach to, or how far can I jump with a SH (maybe to jump over a Falco laser for an approach)
Movement- Should be able to WD/WL, needle cancel, shino stall, and just generally navigate platforms on all stages. It also can be helpful to know which platforms are low enough for you land on with a SH
Punishes- Sheik's combo game should be fairly simple to learn, and not too difficult to execute, but it still takes time to develop a 6th sense for how to comob, but in the long run, you should not really be using much brain power on how to combo people
As for how to think like a Sheik, I would just say the main thing is patience. A lot of people nowadays are saying that Sheik is more of a defensive character, and should be approaching much, but more baiting out attacks and counterattacking. It's kinda hard to tell when you watch high level play, but often times, this is how the Sheiks get their hits in.