I've made many bargello quilts, and I love both the process and the result! Here's my best advice:
1. Buy a whole lot of thread! (When I first started making Bargello quilts, I wished I'd bought stock in a thread company:-)
2. Wash, dry, and starch your fabrics before you even think about cutting them.
3. When you iron the starched fabrics (before cutting them), be careful to do so along the length rather than the width of fabric.
4. Understand that the first step of the larger process is to create a new fabric. Allow it the time it deserves. The final result is soooooooo worth it!
I hope this helps. Please feel free to PM me if you have further questions.