There are several aspects of quilt making that must be done in order for a quilt to be flat and not have wavy edges. In this post, I talked about how I cut my border strips. The border strips must be cut straight and the borders must be the exact length as the fabric edge to which they’re being attached. If you simply place a long, not measured strip of fabric onto your quilt and sew, then whack off the extra, and you’re getting good results, you’re very lucky. That method doesn’t work for most of us. There’s also more information on measuring and sewing borders in my newest book, 60 Pieced Borders.
Once your blocks are sewn together but before adding your borders, check your corners.
I find it easier to line the edge up with an inside line on the ruler (and not the outside edge). If the edge needs to be trimmed, I then move the edge lines of the ruler to the edge lines of the top.
If you’ve been careful to measure and square up your blocks before sewing them into rows, there should not be any trimming that needs to be done at this point. If there is, you may be trimming off points that matter (stars) so please be careful about cutting, sewing a quarter inch seam and pressing so that minimal trimming would be needed.
When adding non-pieced borders, I add the sides first, then the top and bottom strips. Once these are pressed, I again check the corners and square them up if necessary.
By following these steps, when your top is done, it should be straight so that you don’t end up with one side that is 88″ and the opposite side being 87″ or 89″ (or worse)!