Need more help with diagonal cutting!!!
 

I don't know how to ask my question, so please look at the photo. See how the stripes radiate out? Some way I did this by accident while working with scraps and now I can't figure out how to do it on purpose. Can anyone tell me how to cut my fabric so that I can get four large triangles like this?

It looks to me like they're cut on the bias. Did you cut one large square then sub-cut into triangles?

Like quiltsRfun said. It looks like you cut a large triangle and cut it along both diagonals to get the 4 triangles. Lots of bias edges, but a very neat look. You need to cut your squares 1 1/4" larger than the finished size of your center square.

Cut a strip that's as wide as one of those triangles is high (from the center point to the flat side). If the stripes run selvage to selvage, then you want to cut parallel to the selvage. If they run horizontally across the fabric, then cut perpendicular to the selvage. Then cut the strip, starting at one corner, at a 45 degree angle down. Starting from the end of your previous cut, cut at a 45 degree angle up. Then down, up, etc. Keep going until you've gone down the whole strip. That's if you need many.

If you only need one block's worth, then cut two squares the same size as your central square there plus twice your seam allowance, lining up the fabric so that it goes along the diagonal of the square. That's usually easy, since the lines will either go vertically, horizontally, or (if you're lucky) diagonally (45 degrees) with respect to the selvage. Cut the squares in half along the diagonal, and arrange them around the outside of your central square to make the block.

I don't think they were cut as QST's (one square cut along both diagonals).

If you were to cut a striped fabric square into quarter square triangles (QST's), you would NOT get all 4 triangles with the stripes in the same directions.

You'd get 2 vertical and 2 horizontal striped triangles.

x7lillies's guess is probably right (either method) - but the problem (ok, maybe not problem but issue) is the way you've attached them to the center square.
Both ways she suggested end up with bias on the outside.

I would suggest making sure that you really starch that strip or square BEFORE you cut it to help stabilize, and try to handle, stretch and mangle the sewn square as little as possible.

While I hate working with exposed bias (even though I know how to control it), sometimes you gotta to what you gotta do in the interest of aesthetic design.

Are those striped and plaid designs from quilt fabric, or shirting fabric?
It's a great look.

Take a square of paper, draw some lines on it & play with that, sorry don't have any other wonderful advice.