If I were making this pattern, I would start by starching my fabric heavily (before cutting)
because you are dealing with bias edges here which tend to stretch a lot...hence your wavy
edges.
Then I would pin or better yet use tiny dots of washable glue, heat set then go the
sewing machine.
Hate to diss Joanie Doan but she makes all her tutorials look so easy when they aren't
unless you are an experienced quilter.