One of the secrets to making your finished upholstery piece look more professional is making relief cuts.
Below is an example of relief cuts. It shows welt cord being sewn around an outside corner. Without making the cuts, as shown in the picture, the welt would kink and the fabric, to which it is being sewn, would pucker.
The picture shows the welt going around a curved corner. Whether it is a curved or 90 degree corner, you still need relief cuts.
Usually, 3 relief cuts will work, but sometimes you may need 4.
When making the cuts on welt, be careful not to cut into the thread that sews the welt together.