You can get safety pins that have a bend in them. The bend is so the come back up easily. Or you could to a basting stitch throuth the layers. Start in the middle when quilting so that if the layers shift you do not end up with a pucker in the middle of the quilt. You can smooth the layers outward as you go in needed. To mark your quilt, there are may tools. Water soluble pens, air soluble, chalk. Use a ruler to mark evenly.
Good luck and welcome to quilting.