The first thing you need to do it obtain data about the speaker you are mounting. For most subs, this is often given, but it's easy enough to measure yourself anyway. The following numbers are for the sub shown in this picture. The names in brackets will be referred to below.

Outer diameter [OD] (edge to edge of sub): 10"

Inner Offset [IO] (distance from edge of speaker to mounting edge): .375"

Inner Diameter [ID] (outer diameter - 2(inner offset), or measure): 9.25"

These are the two rings that you are going to make. The rings here are flush mount rings Meaning that when you mount them, the level of the subwoofer will be flush with the wood, not popping out on top. If you dont want to make flush mount rings, you can just make the ring on the left.

The ring on the left is what the sub will be mounted onto. It's inner diameter is the same as measured above, ID. The outer diameter will extend past the edge of the sub, I usually choose 1" over the measured outer diameter (OD), -- so 11" applies here.

The right on the right is the flush mount peice, and it will sit on top of the ring on the left. This ring's inner diameter is the the measured outer diameter (OD). And this ring's outer diameter is the same outer as the first ring, so 11". The flush ring is for cosmetics only, and needs only to be as thick as your speaker. For subs usually 1/2" works well. For speakers/tweets, 1/4" works nicely.

Now you just need to trace and cut your rings. For making nice circles, you can simply use string or a peice of ribbon with the center nailed down, then measure the radius, attach a pencil and go around. For example, if you want a 10" circle, make a 5" long peice of ribbon or strip of wood, and go completely around. If you use a router / spiral saw when you do the actual cutting, make sure you cut the outer diameter first, so that you can use the same center pivot point for the next cut. Here, I used a rotozip to cut the wood.

This is the two cut rings, same as shown in the diagram above.

Now you have to glue the rings together -- thinner one on top of the thicker one. Use liquid nails or any good wood glue, and let it sit.

When gluing, try to apply even pressure. If you have a spare board lying around, put that on top to distribute any weight evenly.

This is how it looks once the speaker is mounted. Note: Here the flush ring is 3/4" MDF and it actually comes higher than the speaker. If i had used 1/2" here it would sit perfectly.

You can see the use of these rings in nearly every fiberglassing tutorial on my page.