Find the Photos app and open it. You need to have photos in the app already.
Double-click on one of your photos, then click on the Edit button in the top-right corner of the window.
From the items on the right side of the window click on Extensions, then select More…
Enable Super Borders by ticking the checkbox beside its name.
Once you have completed these 4 steps Super Borders is always available from the Extensions in Edit mode.