I trim my back and batt about 1/4 inch past the quilt edge. Depending on the size, I will use my table or the floor to spread out my quilt full size. I then use my large squares and rulers to square up the sandwich. I put a large square on the top corner making sure I will not be cutting off any fabric needed for a 1/4 inch seam when I attach the binding. I place my other large square at the bottom corner. I then line up my long clear rulers to connect the 2 squares. If everything looks good, I draw a cutting line with my Frixion or chalk pen. I do all 4 sides this way and then bring my quilt to my cutting table to rotary cut on the drawn lines.
Everyone has their own method for squaring up their quilts, this is just how I do it.