Have you already cut them? If not then put a row of stay stitching on all the bias edges before cutting them. If they are already cut, I might cut strips of tissue paper and stitch, stay stitching along the edges through the tissue paper. If you pin the bias edges to the paper, you will be able to tell if they are moving and correct them. Bias edges are not that difficult to manage if you use lots of pins and can pin them to a straight edge or border.