Fabric covered welt should always be made with strips of fabric cut on the diagonal, or bias. The reason is that a bias cut gives and stretches as needed, and welt usually has to curve and wrap around corners to follow the lines of your project.
Your strips don’t have to be perfect, but make sure they are wide enough to fold over your welt cord and leave you with a 1/2 to 1” lip. I tend to like a small lip, but you may want a full inch to work with if you are new to upholstering.
