How big are your square units after you sew them?

You did pick a challenging pattern for a beginner - not impossible - but the units do need to be the "right size" in order to fit together properly.

So if I remember correctly, your squares need to 7/8 larger than your finished unit with that method. Is that what you are doing? Someone correct me if I'm wrong about that measurement.

According to the size thread you are using, you may need to be making a scant 1/4 inch seam. Are your HST's too small or out of shape or what?

This is correct, but I always start with my squares 1" larger, then trim my HST squares to the correct size.

I recently bought an Easy Angle ruler and find that I have good luck with it. With it, you cut both strips WST to the size of the finished HST. Then use the ruler to cut the triangles, rotating the ruler to use up the fabric. Then sew a consistent 1/4" seam on the bias edge. I used to do it your way and love the fact that I don't have to trim anything anymore.

Make them slightly oversize then trim to actual size.

This is exactly what I do as well. Never get them the right size any other way.

Oh, and I forgot to mention (though others did mention it) the Tucker Trimmer is an awesome tool to have for HST. A regular square up ruler will get the job done, but the markings on the TT are easy to read and just simplify the process for me.

There are lots of ways to make HSTs, but whichever way you use, don't try and cut the pieces to size before you sew. Cut them larger, then trim them down. When you trim, make sure that the seam is exactly on the 45 degree diagonal. It should go precisely thru the corner. The normal method is to start with squares 7/8" larger than your target size. Most folks round up to 1"' but if you are a newish quilter, I would make them even larger, adding a quarter inch or more. As your skills improve, you can scale this back. Hst tools are great, and I second the mention of the bloc loc rulers. A bit pricey, but worth it to me

Yes, this was one of the first rulers/templates I bought way back when, and it is a very good way to make hsts if you are making quite a few out of the same color combo........