I've never made a shirt from bandanas before but I have made a backless top. I <3 backless tops!!
Basically I took a bandana and if you are small enough you can just fold it diagonally, to make a triangle, then tie the two ends behind your back. If you need more room then you could attatch a couple of ribbons to the ends?? (I don't know about the ribbons, I just tied my bandana around the back, I had a reasonably large bandana!!)