The second photo shows after stretching but before trimming, the third photo is a different one, but after trimming. You just need to go by the lines on the mat, nothing else is square!
Stitch it all the way around the edges of your panel, with the folded edge toward the center, and with anything less than a 1/4" seam allowance...this is just to tack it on. It will get stitched accurately with the border seam. Cut each end even with the panel, no extra is needed. It doesn't matter what order you do them either, I just go around.
The panel in the last photo is horribly wrinkled too, even after pressing, but I knew the free motion stitching would disguise that! See the finished quilt at the top of the post.
Then I stack my little quilt top with batting and muslin, about 17" square. You could add the back now instead of the muslin if you want, but I like to cover up the stitching in the center on the back.
Quilting the center, using the design as a guide for patterns and colors. Have fun with this! I usually just outline the main shapes, so they puff up a bit. Then I add the back and quilt the borders. You could do the border quilting before adding the back if you don't want any quilting to show on the back too.
Congratulations, you are done! Let me know if anything is not clear, I'm writing this really fast, ha!
Happy sewing, Beret