I’m sure there are other ways to do this, but here’s my take! (Designed Dress 7 to be able to answer this question)
Cut a piece of elastic to desired length, then pin one end to your fabric (I do it on the wrong side of the fabric), stretch the elastic along the fabric, making sure the fabric is flat, and pin the other end. While the elastic is stretched out, pin along the line as much as you’d like.
I just put three other pins in between the two ends for even distribution.
It gets a bit tricky sewing it. As soon as you have one end sewn on, make sure you’re stretching the elastic so the fabric is flat and pulling the sewn end outwards or you might end up pulling the fabric your way (no buenoo). I prefer using a zigzag stitch, but it’s really just up to you(: Don’t forget to take out your pins along the way! 
Hope this was helpful! (Let alone, made sense).. Hit me up for any further questions. Posting Dress 7 later tonight! :D
