Identify the center point of the side of your cabinet. An easy way to do this is to pull a chalk line from one top corner to the opposite bottom corner and snap it. Do the same for the other top corner. The point where the two lines intersect is the middle.
Center the hole saw over the center point you created. The size of hole saw will depend on the dimensions of your speaker. The hole saw should be the same size as the diameter of your speaker.
Open the cabinet so you have access to the interior. Staple a piece of black speaker mesh over the hole. This will cover the actual speaker to keep it from getting dirty and create a more aesthetically pleasing look on the exterior of the cabinet.
Spread a layer of epoxy on the front of the speaker casing. Make sure not to get any on felt paper or it will disrupt the vibration of the speaker and cause a loss of sound quality. Press the speaker up against the mesh so that the speaker completely covers the hole in the cabinet.
Drill a small hole in the back of the cabinet to thread the speaker wire through. Wait for at least 24 hours for the epoxy to fully cure before using the speaker.