There are a couple of ways to achieve that. One is called an Attic Window block, that has a diagonal seam along the points. It is often done in two tones to give a dimensional effect, but can be done with one color. Many many free patterns and tutorials on Attic Windows. The other thing would to look on how to miter, same thing -- it's often how we do borders.
The other common way would be to "frame" it, or I call it "Sashing as an L". One side of the bars is the same as the square. The other rectangle is longer, it goes over both the square and the other bar of sashing.
You can get some fun out of stripes by strip piecing WoF (width of fabric) a wider strip to a narrow one (or have a narrow one top and bottom), cutting as a triangle and then resewing along the long diagonal through all the pieces. If you cut the stripes well, they will miter together when you do the diagonal.
I used the attic window setting earlier this year, in my case I made rectangles of postcard sized large scenic prints. The attic windows are the two tones of grey. The black sashing was put on like an L, so both in one project. Here's a picture over here
2019 Fabric Moratorium