I learned a lot in my first quilt class years ago, but now I want to take steps toward improving. One thing I never learned is how to square up a block. It seems like all the books just tell you to "square up your blocks", and assume you know how. I kind of get it, but I just don't know where to begin the process. Do I find the center of the block and go from there? Pick an edge that looks straight and work from there?
New here, so forgive me if this has been answered many times before.

You want to start from the center and have all your edges even from the center. Did that make sense?

Here is the way to do this you get a ruler with the 45 degree line on it. you line up the center of the block with the line

Line up the center of the square. checking the rights side and the top line up the edges top and side . The check the other side before you cut. Mearsure twice cut once

I have 4 1/2 inch, 6 1/2 inch, 12 1/2 inch and 15 1/2 inch square rulers. I usually use them to place on top of the block and trim around the edge.

What if the block doesn't have a true center, like say a Sunbonnet Sue ?

I use my square rulers and put them on top of the block, center the applique (or whatever) and trim around the outside.

Then you pick what you want to be the center. Usually you can pick the part of the doll that you think is the center and leave blank space all around. I use block rulers also. Center you picture and trim to size. Lots of blocks are 12 1/2 inches so a ruler that size is very useful.

This might be helpful

http://www.quiltuniversity.com/squaring_up.htm

You can google the topic "squaring quilt blocks" and see what else might come up

I put a little red dot on my sq ruler to find the center easily. Then I slice around the block.