Hello. The pants are going to be difficult. They are basically the same as Sora's pants from Kingdom Hearts. Most people would use elastic at the bottom of the pants, which will make them poof a bit. use a nice thick fabric though or it won't sit correctly. It may not be what it SHOULD be made out of, but it will give you the look you want better. I'm thinking a bottomweight should work just fine. If you want to go with more correct fabric (thought I cant place te name) you could use wirein the bottoms as well as the elastic. Not sure if it'll show or not though.
As for a pattern, I can't help you with which one (maybe someone else can), but you can use maybe sweatpants type patterns and alter it. You would cut them wider then the pattern and make them shorter too.
Can't help with the hat though sorry there..
Cosplay DA Regular DA
Kumoricon 2014 (so far):
-Sora (Kingdom Hearts 2)
-Serena (Pokemon Y [It's the one I have lol])
-Akaito (Vocaloid. Black & Red Ver.)