My Merida dress is finished, so I thought I would share the process with you. I also recommend looking at Angela Clayton's website to see her guide on making the costume. She explains her process in great detail, and provides information on making Merida's cloak and hair.
Merida is easily the best female character (apart from Mulan and perhaps Yzma XD) that Disney has created in recent years. She's brave and tough, and her story doesn't end with her falling in love - instead, she matures, while retaining her free spirit. I think she's a great character, so I wanted to cosplay her.
Once the fit was right, I cut out the dress pieces. There is 1 front piece, 2 back pieces, and 4 godets for the sides to add volume. A godet is a triangle of material that you add to increase width: the opposite of a dart.
I added eyelets to the back and cut the little V shape out of the front.
green sleeves are one solid piece, then ruffled pieces of white cotton were sew on top. Next I added the green rectangles on top.