First and foremost, SLOW DOWN.
Start with whatever is furthest away from the viewer. Shaping is the most difficult part of Intarsia to learn. A lot of it was trial and error for me. when you have multiple pieces that need to be shaped together, use double sided carpet tape to attach them to a piece of scrap plywood, and then you can sand then all at once. Once you take them apart, you can fine tune any shaping that you need to do. Take your time, and try to envision what the subject looks like. Use photos to compare and study what something looks like in reality, and then you can slowly shape to that look.
I highly recommend a good beginning Intarsia book, that has step by step instructions for simple projects. Judy Gale Roberts has a couple good ones, "Intarsia Workbook" and "Wildlife Intarsia" are both great starting places. You can get them from her site.
http://intarsia.com/shop/product-category/books/intarsia-books/Kathy Wise has a good instruction book, but it is geared to a more advanced user.
Don't be afraid to post pictures, The feedback you can get from folks on here will make your work better.