Step 1: Backout the nut from the side bite fin box, but not all the way, just enough so the fin can slide in without hanging on the screw nut.

Step 2: Slide the fin into the back notch and lower the front in.

Step 3: Tighten the screw so that it's snug but not too tight to prevent stripping. Depending on the type of paddling you are planning on doing determines whether or not you need them. In the surf it will pull your board around and turn easier, and on flat water, it will keep you moving straighter.