I have been wanting to make 'fabric' out of plastic bags for a long time and I have read a lot of tutorials on the best way to fuse bags together. You can read about how to fuse bags together
here,
here and
here . My advice would be to practice, practice and practice - experiment with the iron temperature and with a different number of layers etc. I still need some of that practice I'm recommending!
For my recycled bag holder I used Target brand bags. I cut the design off the front and just used the white portion of each bag. I layered the bags in the following pattern to make a larger piece of material.
I repeated this layering process until I had used a total of 12 bags - 6 layers of plastic at each corner. I used one of the logo fronts on top of 4 other layers of plain white bag to make a type of binding tape for the project. Once all my bags were fused together I trimmed the edges and was left with an oblong shape.
I cut the binding tape 1/2 thick on the diagonal - sewing it together (and onto the bag holder) using a zig-zag stitch so it would be more decorative.
I folded binding tape over top and bottom of the oblong and secured it using a zig-zag stitch.
I placed a piece of binding tape at 2" down from the top and 3" up from the bottom. I threaded elastic through each of these - pulling the elastic tight and securing at each end. I placed a hoop of binding tape at the back for hanging the bag holder.
After sewing up the side edges - it looked like a large cracker!
Bags go in at the top and you pull them out one by one at the bottom.
A simple neckline for easy on and off.
Three growth tucks so the nightdress will last a long time. 
I'm pleased with the results - and I dearly hope they fit both girl and doll well.
