There's more than one way to skin a cat, but that's a different page on the wiki. 

There are many different ways to attach fins to a rocket. This page will review the three main techniques, and when it is best to use them.

Tab-Slot Fins

Tab-slot fins, also known as tabbed fins, are the most common, reliable, and lowest-cost way to attach fins. It is done by cutting a slot in the body tube of your rocket, and putting a matching tab at the base of the fin, then connecting them with a glue or epoxy. This attachment method ensures the fins are straight and retains them accurately. To further reinforce the fins, a matching slot can be cut into motor centering plates that can hold onto the fin tab. 

The downside to tab-slot is that the fins needs to protrude far into the rocket, and the fins cannot be removed without destroying the rocket. If you ever break a fin or need to change the geometry, too bad!

Example: The tabbed fins on Lightwave 1. 

Bolt-On Fins

Bolt-on fins are fins that are able to be removed with a mechanical attachment mechanism, like bolts or screws. They eliminate the permanence of tabbed fins, but do increase the weight/complexity of the fins, and severely limit the maximum fin flutter, since the hardware can not handle resonance as well as a tab. Sometimes to prevent air pockets, you may need to use some sort of temporary glue or sealant around the base of the fin, like single-part glue or liquid gasket. 

Example: The bolt-on fins of IREC 2019 "Water Bearer."

Surface-Mount Fins

Surface-mount fins, also often called flush-mount fins, are fins that do not interrupt the body tube of the rocket. These are very difficult to design and implement, but do offer the advantage that you can use the entirety of the inside volume of the body tube. They are rarely seen outside of minimum-diameter rockets. Usually they are applied and secured by wrapping the body tube and a plastic fin core with sheets of carbon fiber and curing it.