Oh, it's quite a production! I lay my quilt out flat on the floor. I take all my rotary rulers and large 15 inch square ruler down there with me. I put the large square in the corner and line up my rulers end to end with the square down one side of the quilt. I draw along that line with white chalk and them do the other 3 sides. If everything looks good with a good 1/4 inch seam allowance for the binding, I put my cutting mat under the quilt and use my rulers to trim the edges.
I have knee pads that I wear for working on the floor. There are laser lights and quilters chalk lines you can buy to make the job easier but my way is cheaper.