I would take the top piece apart. Trim the multi colored triangles just a scant amount on each outer side as I see them here. Then I would work them into the larger piece by pinning. Remember also, if one side is longer than the other, put that side down towards the feeddogs on your machine. That will take up some of the slack. Also, I would use a Walking Foot attachment, if you have one. That helps to work the bottom piece in better. It is a very pretty square by the way. Good luck.