Help to square up a square
 

I tried uploading a pic of the square but it keeps failing.It is only 1.43 mb in jpg format so not too large. Regardless, I am making a double pinwheel block, it has 4 squares of one large triangle with 2 smaller ones sewn to the long side of larger triangle. These squares need to be trimmed to size and squared and I am having a hard time getting them right without compromising the design created when all sewn into the block. I believe the square has a "y" seam if that helps to define it. Does anyone know what I'm trying to do? Thank you in advance!

It's hard to say without seeing a picture. That said, when I'm squaring up HST's, I put the 45 degree ruler line on the seam and then trim. That way the seams match what they are supposed to on the next round so the design element shouldn't be lost.

I think slotted trimmers are the cat's meow for trimming up squares and keeping the patch design intact. Be sure to get one that has the diagonal lines crossed in the middle, so you can line up stitching lines perfectly.
https://quiltcut.com/shop/quiltcut-s...14%C2%BD-x-17/

Even better, I use the ruler right after I press the quilted square and it holds in the heat (and steam if you've used steam) just long enough to make the square perfectly FLAT.

It's a joy sewing together the squares I trim with this ruler as they are so perfectly square and flat.

I typically sew large and trim down which requires a variety of techniques but which I feel give me a better finished project. I keep 1/4" gridded template plastic and will make cutting templates, especially if I have a lot of blocks. Warning: Do not use the template plastic as a ruler you will cut into it or have other problems. Lay it out on the block and place your cutting ruler on the piece of fabric to be trimmed off and move the template out of the way.

In some ways easier, I also use quilters 1/4" masking tape on my rulers to show where the diagonals should be. I have a lot of rulers and keep a roll of the tape in my essential supplies.

I find it more accurate to measure a square from the center of the block out. Ex. a 6 1/2 block I would measure 3 1/4 from the center then cut, do the same for each side. The square will be perfectly square.
When I post a picture I edit it to 800 for width and usually that sets the length to 1200 or so.

I think I know what you are trying to make put your Ruler with the diagonal line in the long seam then move it so the half measurements of your block falls at the intersection of the smaller triangle. Trim two sides and turn to trim the other sides

Are you asking how to square up a square block or square up a hst? Two very different to square up.

First, yes I did hit "upload" and it failed 3 times.

I took the ideas for lining up the 45 degree angle and using the point where the smaller triangles meet as the center and it's working perfectly! Am working on them as I type this out. Thank you so much for all your help! You are all the greatest!
I looked at that video and yes, those are my squares. I knew you'd know what the heck I was trying to explain even without the dang pic! 😁

Are you sewing double pinwheel blocks with a y-seam? There is an easier way. Just 4 HSTs attached to 4 larger triangles and sew everything together. One of my favourite blocks!
https://www.generations-quilt-patter...ilt-block.html