Can anyone give me tips on how to square up a very large quilt? Thanks

What I do is make sure the corners are perfect 90 degrees. I use my DH's big tee square. If the corners are perfect then I don't worry too much about the middle edges. :D

I measure across the diagonals, if they are the same then the quilt is square. If not I keep pulling it gently along the sides a little at a time until they are and (if necessary I pin it in place) then leave it to dry with fans on for a day with wool and polyester battings, two-three days for cotton depending on how humid it is.
If it isn't a show quilt, but it looks square anyway, then I don't bother.
HTH

i do the same as sally but i also measure thru the middle both lengthwise and widthwise.

most of my quilts are contemporary works so all i need to see is that everything is the same measurement.

I square as I work on the blocks and then when it is assembled, I measure across the middle both ways. The ends need to be the same as the middle. So far, I haven't had much trouble.

Well .... if there's no quilt police.... I square each block then once the blocks are joined... I measure each side & in the middle & take an average & make my border or binding that measurement. I do my own quilting so not sure how a professional machine quilter would look at my way of "squaring"...

Thank's everyone for the tips. That is pretty much how I did it, just wanted to be sure I was doing it correctly. You are a great bunch!

i've trimmed a tiny bit off corners of large quilts before binding. if that's what you mean. i use my largest runer and place it in the corners and trim if necessary. and i have laid out smaller quilts when wet and measured across and diagonally.
good luck

Umm - if everything is cut, sewn, and pressed correctly, why is it necessary to do a major "squaring up" of anything at the end?

Before adding borders, I do measure the body of the quilt (both directions) and then use the average measurement for the borders. I will ease the edge pieces (the row before the border) as necessary. I mark the border strip at appropriate intervals to help with the matching.

That said, after putting the top on top of the backing, I do check it to make sure that I placed it correctly. I use a 16 1/2 inch square on the corners and then run yardsticks along the edges to make sure that it is laying straight.