Here’s a few pictures, to show how I square up my blocks, without fancy square rulers in a million sizes.
Here we are with our HST. Find your 45 degree line, then figure out which side of your HST is already the most even or square. (See 
below for getting them square to start with. See 
for if both sides are wonky & you need to trim all around.)
I tried it each way, and discovered that it was pretty darn square with the yellow at the top. I want the left edge, and bottom edge to be along an inch line, and the diagonal seam along the 45 degree line.
Next, find the line on your ruler for the size block you are making, and line it up along the left edge of your block. I’m cutting these HST’s down to 4.5", so you see my index finger at 4.5" on a 5" ruler.
Trim away anything to the right of the ruler.
Now leave your block in place, but turn your ruler. (This photo was taken from the same perspective as the previous ones.) Line your cutting size line (4.5" here) up with the bottom edge of your block.
Trim away anything above the top edge of the ruler. (Or walk around your table if you’re able to, and cut from there.) Finally, trim off that last dog ear. You now have a 4.5" HST, without needing a special and stupidly expensive 4.5" HST ruler.
For those of you struggling with getting good matching points on your finished blocks, my hint is this:
Getting your HST’s to be pretty square to start with, giving you a straight side to cut from when squaring up, is invaluable. To do this, be careful with your opening and your pressing. If possible, you want the seam allowance to go towards the darker fabric. So here I’ve laid down the block with the yellow on the bottom, and flipped the black upwards.
Start by lightly finger pressing the block open. Press straight down with your fingers. Don’t run along the length of the seam. That seam is on a bias, and it WANTS to stretch out of shape. Running your fingers along that seam will encourage it to do just that.
Now use a dry iron, and again, press straight down. Finally, now that it’s pressed open without being stretched, give it a blast of steam.
If both sides of your block are wonky, and you need to trim all the way around the square, the method is pretty much the same as above. You’ll simply be cutting a little less off all 4 sides, but will still align your diagonal seam with the 45 degree angle on your cutting mat. Apologies that the photos for this are also a yellow & black block. That was the only large HST I had at hand to demo with. For this section, I’m cutting with the black to the top, to try to keep them clear.
Here you can see that while my diagonal seam matches the 45 degree line on my mat, but my left and bottom edges do not match up with an inch line. They are intentionally placed a little below and to the left of the lines, so I can cut those edges off later.
Now set up your first cut. Line up the measurement you want to cut to, along the inch line on the cutting mat (not along the edge of your wonky block), making sure you keep that diagonal seam on the diagonal line. I’m cutting to 4.5", so my finger is pointing to the 4.5" ruler line, which placed on top of the inch line of the mat.
Trim anything off the right side of the ruler.
Turn the ruler and do the same along the top of the block. Make sure to keep those diagonals aligned.
Now that your right and top edges are straight, you can go back and do your left and bottom edges, this time aligning your ruler edge with the inch marks on your mat.
Chop off everything to the left of inch line.
And everything below the inch line.
You now have a perfectly straight block, no wonk in sight.
