There are a couple of ways that squaring up your quilt can be handled...as quilters are all different, of course there will be little things here and there that we each do differently. If you will be blocking your quilt beforehand, you will handle this aspect just a bit differently than I show you here.
For those of you who are simply going to trim and bind after the quilting has been done - here is how I square up my quilts (a service that is included in the cost when you send a quilt to me for long arm quilting), to be sure that I trim away the excess backing and batting without distorting the shape of the quilt.
Squaring Up Your Quilt in Preparation for Binding
Depending on the density of the quilting you choose, you may opt to block your quilt before binding. Blocking a quilt is simply wetting the quilt and in essence, stretching it back into it's original size and shape, and then letting it dry that way so the new shape becomes square, crisp, flat, and for the most part permanent. The results of blocking will last for as long as the quilt lives - or until you wet it again. So a blocked quilt is not one you would want to ever fully submerge in water again.
Often times with quilting our quilts tend to shrink up a few inches, and even more than a few inches when you get into very heavy, intense heirloom quilting. Reasons that you would want to block your quilt before trimming away and squaring up would include wanting the quilt to hang straight in quilt show or on a wall or extreme distortion of the quilt due to heavy quilting. For most quilts that are going to be used on a bed or sofa, blocking is not a necessary step - but I mention it here because it is a step you want to perform before squaring up your quilt and trimming away the excess backing and batting.
I like to use my dining room table, clean and washed of course, that way I can use my dining room chairs to support the bulk of large quilts, while the area being cut is laid flat along the length of my table.
Click on the image to enlarge if it helps you to see - here I am use a 16" square ruler along with my 24" long by 6" wide cutting ruler. I usually begin in a lower right hand corner, just as you see above.
I am not concerned as much about the rest of the quilt for now, just what is under the ruler. The cuts at the four corners of the quilt are the most crucial to remain square - as long as you succeed here, it will create a guide for you to use throughout the rest of the quilt.
Once you butt those two rulers up to each other and line up the edges of the top with your rulers...you will look closer and notice something like you see in the photograph below. Areas of the quilt will line up nice and flush with the rulers...and then you'll have places like this one!
If I were to leave these areas and not correct them now, not only would my quilt not be square when I'm finished, but I also leave the risk of having an area where batting would show and the edge of the quilt would not be caught in the binding when it is attached. So what you want to do is line the majority of the edge up with the rulers - and then gently tug on the quilt sandwich (that means pull gently on all three layers) and pull those areas flush with the rulers. You are just very lightly stretching your quilt back into its original shape.
Sometimes I really have to give a good tug like this picture below shows, to get things aligned correctly. You really want to be sure that this part is nice and neat, as it sets you up for the rest of the quilt.
See now after a little pulling and prodding everything lines up nice and perfect?
Now that everything is nice and flush with the rulers, it's time to cut! Note that I am not cutting any away from the quilt top - but rather, trimming away the batting and backing fabric so that it will become the same size and flush with the quilt top.
Now this was Cindy's beautiful and LARGE King Sized quilt - so that means there is a long way to go from each corner to corner. For a quilt like this, that I don't expect to be hung, I would not use the square down the entire length - what I would do instead is keep using my long straight ruler until I get to about the center of the quilt, then grab my square again and using the piecing as my guide, I make sure we're still square and nothing is getting out of whack.
After a check with my square ruler I continue on with my long straight ruler down the edge of the quilt until I reach the next corner. HERE again, we're at a corner...so whatever I do now is going to set me up for the next side of the quilt! I want to be sure it remains square! :)
I again butt my square ruler up to my long ruler and cut - then I turn the quilt, so I'm again working from the right bottom corner up...and start the whole process over from the beginning until I've completed trimming all four sides.
Another thing to note - there are three ways I generally trim a quilt and I leave it to you to decide what you want for yourself. I personally like my own quilts trimmed flush with the top, but I often will trim 1/4" or 1/2" away from the top for ladies who like a wider or fuller binding. All of that extra would be enclosed in the binding and not seen.
So that is how I square up a quilt! If you have any questions leave a comment and I would be happy to answer them to the best of my knowledge! If you square up a quilt differently I would love to hear your thoughts as well, and what methods you find give you the best results!