The lines on the diagonal (bias) may stretch so just be careful sewing them. You can use painter's tape to mark one line across. My Bernina has a little hole in the back of the foot that you can put a bar in. The bar lets you follow the first line stitching at regular intervals across the quilt. It works well if your machine has that feature. If not, just do more lines of painter's tape or mark with a long ruler with a erasable pen. Always try your pen/pencil on a scrap to check if it's really erasable first.