There's a SIF hops tutorial at
http://www.youtube.com/watch?gl=AU&h...&v=ZjcE4Yc1lCY
How high can you hop seat-in? Most people say that you should focus on seat-in until you can get to around 50 cm, before starting to hop SIF. The reason to switch to SIF is that you can tuck further than you can seat-in.
You can practice that tuck separately from the hop. Just get SIF, balance, and then squat down as far as you can go and stand back up. When you get that to where your butt touches the tire then that's what you'll want to be doing in the air.
If you're just beginning with SIF, then you just need to practice hopping around SIF until you get good at it before trying for height. When it's boring to just hop around, then start trying to hop for height. If you focus on jumping equally with both legs, you should be able to match your seat-in height right away before even adding any tuck. The tuck will just get your tire up that much higher when you add it.