You can purchase stencils from stencil companies on the internet or at quilt shows or quilt shops. You can also use the kind of stencils you use to decorate with paint etc. I use a lot of cookie cutters for interesting shapes especially for children's quilts or holiday motifs. I actually collect cookie cutters as memontoes of my travels that are also useful.
You mark the front of the quilt using the stencils or some object that you draw around. There are special pens and pencils made for that purpose. Some of them fade away quickly and some have to be washed out. You can also mark with powder or chalk dust. Joanns even carries some stencils and the marking tools. You can buy kits for white on white or pastel colored quilts, wallhangings or pillow tops etc. that are already stenciled. You then quilt them and wash out the markings. I have made everything from a pillow to a queen size white on white quilt from one of those kits. The kit often includes everyhthing but the batting. If you use a solid color or white back on your quilt you will be able to see the wonderful designs you created when quilting.
If you don't want to have to mark the quilt top you can just echo the lines of your piecing on the front at least 1/4 inch from your seam lines. The same goes around any appliques etc. I have seen people use a large floral print on other graphic design for the backing and simply quilt around the motifs of the back print. This gives the front a unique look. If I get around to it I will take some pictures and post them for you of some of my quilting. When all else fails you can simply crosshatch the whole quilt. Either in squares or diamonds.