1.) That's totally awesome! I did a Wonderland Kyouya a week ago for my husband but couldn't bring myself to turn him into a caterpillar. I designed a Queen of Hearts costume instead (that everyone seemed to think was from the manga).
2.) I'm sure there is an online tutorial for this, but I couldn't say where. You could honestly do this with quilting cotton and make it pretty good. As for the legs, you will want to make the body squishy with leg holes that have an inset piece of fabric gathered upward to elastic that fits around your calves. The best example of this I can give you is a stuffed Halloween costume you might get at a costume shop (think a stuffed sumo suit). Look at how the leg holes are done.
Definitely make it like a tube that is stuffed and lined with a tube down the middle for your body. Think of pumpkin suits but thinner and longer.
Cons 2014 - Ahn!Con | Kawa Kon | Wizard World St. Louis | Costume-Con | Marcon | Cosplacon | Dragon*Con | Archon | Chicago TARDIS
In Progress- Cecil from Welcome to Night Vale
| Jonathan Strange from Jonathan Strange & Mr Norrell
| Madeline Westen from Burn Notice
Misha's Costumes in Progress- Mr Norrell from Jonathan Strange & Mr Norrell
Kawa Kon: Into Darkness in 2014!