I've taken some photos of my steps on a recent quilt. This is the method I use most often, and that works best for me. This is NOT the only way to do this, and not even the only way I do it, just my 'go to' method.
Before loading, I measure the quilt across in at least 3 places (just like applying a border) to find the smallest measurement. That is the 'width' of the quilt and I will adjust it all to that.
To make sure the top is square to the leaders, I measure down from the edge of the leader (in this case my red snapper clamp) to the first border seam...here it's 7 1/2". I had already pinned this before I thought to take photo, but usually I'd measure (both 'width' and distance from top) and then pin. You can nudge or tug the fabric up and down a little to make that seam line up.
At the bottom of the quilt, I once again check that the border lines are square.
And here's the quilt, finished and washed.