I recently found t-shirt dresses on Pinterest and I just couldn't wait to try them. I have tanktops that I've been meaning to appliqué for my girls but just haven't done it. I got the tank tops on clearance at Walmart for $2.50 and the fabric is $3 a yard also from Walmart. I bought 2 yards each of the fabric because I didn't know how much I would need.
For the fullness of the dress, I measured their waist and multiplied by 2. For the length, I just cut the fabric at the fold and used that, which worked out - not too short, not too long.
This tutorial said to use elastic thread, which I did. The dress wouldn't have gone through their torso if I had just used regular thread.
For the sash, I just used my measuring tape as a mock sash to figure out the length. I then just cut 5 inches thick (I wanted the sash to be 2 inches thick).
For the flower, I used the mouth of a glass to trace a circle and cut several pieces. There are a lot of tutorial for fabric flowers online.