I usually measure my quilted top before I do any trimming, just to get an idea of how even (or off) the borders are.
Then I clean off my table and lay the quilt with one corner on the cutting mat. (gotta start somewhere, right?) I use two rulers for the squaring process: A large square one, and a long rectangular one. I lay the square one on the corner of the quilt and look at the lines going in both directions. Usually, I find a straight line on the border to get my bearings because it is important to me to have reasonable symmetry.
When I have picked my "guideline" I simply trim the edge in one direction. I generally leave the top and slide the cutting mat along the edge. Use the edge of the already cut part to align the ruler for the next piece on the straight-away. As soon as you near the next corner, I switch back to the square template. Repeat til you have all 4 sides done.
It's a good idea to zigzag the edges down -using the walking foot - before adding the binding, but you don't have to.