Square up the same way. However, you need to have something to balance the weight of the fabric otherwise it will pull the area you are squaring. Also, if you are cutting that whole piece of fabric up, you will probably have to stop and re-square a few times.
Hi! That is made by Olipfa. I found them at Joanne Fabrics. They only come in the size for the 24 1/2" ruler but I had my husband cut down others to fit my smaller rulers. I'm not sure they sell them anymore though, sadly. When I replace my rulers, I'm going to have to find another solution. It's made of a heavy duty plastic.
My rulers are Creative Grid brand that I got at my local quilt shop. The ruler safety shield is something I added. You can get them here: amzn.to/41xkS5i