vivientan

I'm currently working on a Hungry Caterpillar quilt for my daughter. According to the instructions, I'm supposed to cut two huge quilt panels measuring 20 1/2" by 20 1/2". However I realised that the quilt panel itself is not a perfect square and no matter how I tried to square it to 20 1/2" by 20 1/2", it would end up loop-sided. The corners of the panel are not perpendicular, if you know what I mean. Hence, I've cut it slightly larger than 20 1/2" by 20 1/2" to include the green borders around it. Even then, I'm unable to get a perfect square and had to cut with the green borders parallel to the white panel.

Now that I'm ready to piece the quilt together, I'm worried about how the quilt may end up. When I refer to the pattern of the quilt, the green borders don't appear at all. How should I trim the panel, in order to get a perfect 20 1/2" by 20 1/2" square? And sew the border to the panel so the green borders don't appear?

Any advice here will be much appreciated!