Square-in-a-Square blocks are like these:

16 Broken Sash
Broken Sash

18 Economy

In the Patterns

You will be given an accurate measurement for the centre square, and information for triangles, cut on the diagonal. The triangles will be generously sized, so you can trim the block accurately at the end. For example:

I prefer to cut the squares only once on the diagonal, to avoid having the bias on the outer edge of the unit. This means it won’t stretch when you join the unit to the rest of the block.

Later, in the instructions, you will see something like this:

This means add the triangles, with a scant ¼” seam, to alternate sides. Press. Then sew the other two triangles, to alternate sides. Press.