If your quilt top is out of square, you'll need to block it. Sharon Schamber has an excellent YouTube video on blocking. It is the method I use.
Here is the procedure I use: I float my top and batting and have done away with the center bar when doing bed sized quilts. First find dead center on all of your leaders and mark the spot. With your backing fabric WRONG SIDE UP, find dead center on the top edge of your backing and pin this spot to the mark on your leader. Use the top bar (through the neck of your machine) as a pinning guide to be sure the top edge of the backing is even all the way across. Place your pins horizontally or parallel to the pole. Now smooth and pin from the center out to the left edge of the backing. Next go behind and smooth and pin from the center out to the other edge (it's easier to pin from the back when going from center to right). Next I pull my backing out over the belly bar and check the bottom edge of the quilt to be sure it lays evenly across the floor from side to side. If everything is even find dead center on the bottom edge of the backing and pin it to the mark on the belly bar leader. Pin as before from the center out to the edges. Roll the backing onto the belly bar making sure to keep the outer edges even as you roll (Very Important!) Next spread the batting out evenly across the width of the backing and start pinning as before but about an inch below where you pinned the backing on to the leaders, again pinning parallel to the top bar. Spread your batting out smoothly across the backing and allow it to dangle over the belly bar and onto the floor. I always sort of kick mine under the frame so I don't step on it and tear it. Do the same with the quilt top, again pinning the top edge about an inch below the batting pins. Smooth over the batting and let the remainder drop onto the floor. The pinning is what makes it necessary to have your backing and batting a few inches larger than the quilt top. The sides also need to be a little larger because as you quilt the stitching will pull everything into the center a little bit and you don't want to end up with a top that is larger than the backing.
I sew a line down each side as far as possible and about an eighth of an inch in from the quilt top edge to hold everything more solidly. Each time I roll the quilt to the next section I sew a little further down the sides. When attaching your side clamps for tautness, only clamp them onto the backing fabric so you don't stretch and tear your batting or stretch out your quilt top. I use two paint sticks on each end, one on top of the backing and one underneath. I attach my clamps to the paint sticks and it gives a more even pull to the sides. Use two more paint sticks on the opposite side. Keep your top roller about a 1/16 to 1/8 inch above your machine base as you quilt. As you begin to roll the quilt up onto the top bar you will need to adjust the height of the bar each time. Have fun!
I hope this is helpful to you and more clear than mud, lol! If you have questions you can PM me.