Can someone help me figure this out?
 

I'm not good with triangles. How do I attach the blue triangles to the yellow square so that, when I attach the white triangle, the 1/4" seam allowance doesn't cover part of the yellow corner. The blue triangles should line up. I started by cutting the blue square the same size as the yellow square and cutting it into the 2 triangles. I think this is where my mistake is, but I can't get it straight in my mind. What am I doing wrong? TIA!

Your blue square should have been larger. Maybe 7/8 of an inch? I forget the fraction for sure. Anyway, many quilters make the square larger intentionally and then trim to fit after it is cut and sewn. Good luck!
You need to have a 1/4 inch seam allowance beyond the corner of the yellow square in order for the white triangle to fit perfectly. Your blue triangles will provide that when applied to the yellow piece.

7/8 of an inch larger is the correct number to make your triangles big enough... I usually just cut mine 1 inch bigger and trim to be sure.

Yes, as mentioned above, you should cut your blue square (the one you will be cutting in half for the triangles) 7/8" larger than your yellow square. Then your blue triangles will be larger and the right size.

Thank you, thank you, and thank you! I knew I'd get the answer here! 7/8"??? What a strange amount......I think I'll just use 1". :D

Here's another method to avoid those bias edges.
http://bronzewombat.blogspot.ca/2011...gle-block.html

From looking the blue triangle needs to be larger side length of square plus at least 1/2" but 3/4" better then try.i always sew from the side with the stitching line visible and make sure I don't go inside the sewing lines thereby leaving the point.

Cutting larger and trimming to be perfect is always better.

*nodding* For me, that's the ONLY way I can get accurate blocks where triangles are involved. It's not a math or cutting problem for me, it's a sewing skills problem. So until my skills improve enough, I go oversize and trim back, and nobody knows I can't sew a triangle straight! LOL :)

I was just reading an article on making half-square and quarter-square triangles, and there was in-depth discussion of the math involved. It said that for the QST, the pieces should actually be 1.20 something instead of 1.25, and that's why people's squares don't come out right. Seriously? That little bit is going to make a difference? I probably have more variance than that just from my level of sewing skill (not expert, that's for sure).

I think cutting larger and trimming down is always better, too.