There are 2 options for this:
Option 1: Symmetrical Design, but different corners (works well on square quilts)
I prefer to have the full design centered on middle (mark or press a line showing center of each side). I center template over center line & work outward toward the corners. When I've finished my last full design, I will look at how much space I have to the corner & come up with a design for the corners that extends out to meet where the templates ended. Or some people just put a partial design that ends at the cornerstone.
Option 2: Symmetrical Design, but different centers (works well for rectangles)
Start marking just outside of your cornerstones & work toward the center line. When you get to the center, you will only be using part of the template (or find a different template/design that fits). I take a piece of paper & cut it to the size of the gap. Then, I see how that looks using just the center of the design. If that doesn't work, I trace the design on a separate piece of paper & try folding it different ways, using the same part of the design on both sides, until I find something pleasing that fits into the space.