After the fins have been sanded and the body tube has been marked the fins can be glued on. It is important that the fins are straight so practicing aligning the fins with the rocket body may be helpful before using glue.
5.1. Glue the base of one fin along an FL line at the bottom of the rocket. Make sure the fin is square to the surface of the rocket body and aligned with the FL line. Hold the fins in place for a few seconds until the glue can hold the fins in place.
5.2. Repeat 5.2 for each fin.
5.3. Set the rocket upside down on a flat surface or over a pole and allow the fins to dry.