I’ve found that a tile floor, or tile-look sheet flooring works as a great tool in squaring up projects. Align your best edge, or a horizontal element (especially if it’s a traditional pieced project) with the tile lines of your floor. Use masking tape and tape the project down, smoothly, but without undo stretching. Here I started by taping down the uneven top and bottom edges and trimming the sides. Every cut must be parallel with your tiles. Now I’ve re-taped, without letting the project move, just the left and right sides. This allows me to slide my cutting mat under the edge I want to cut. Align the cutting mat grid with the floor, too, and maybe even tape IT down. Then you can use a straight edge to trim the edge as normal, ( or a bit at a time, if it’s a big project.) In this photo I have already trimmed 3 edges and have the bottom left to trim.
Drop me a note at PaperPiecingHeartlandATgmail.com if you are interested in the pattern.