I work with a lot of circular stock. My usual problem exists when you need to put a bolt hole circle in it.
In this case, I need 3 bolt holes in the center to mount to a shaft collar.
I made this at Techshop. http://www.techshop.ws
So, you start with your circle. I coat it in either layout fluid, or in this case, since I am lacking, I covered it in permanent marker.
Now, scribe a line across the circle. Doesn't matter where, actually it's easier if you aren't near the center. (my scribe marks are not showing up well on this picture)
Step 1: Bisect the Line
Now we go back to high school geometry and bisect the line.
I'm sure there's an instructable out there that tells you how to do this, so I'm going to give you the cliff notes.
Radius larger than 1/2 of the line length, make arcs from the end points (where the line meets the edge of the circle).
Now, the arcs intersect in exactly two locations.
Step 2: Draw Bisecting Line
Draw a line all the way across the circle, passing through these two points.
This line divides the circle into two equal halves.
Step 3: Bisect Your New Line, Finding the Center of the Circle.
Repeat the previous steps, on the new line.
Where the line you are bisecting, and the bisecting line meet, is the center of the circle.
I know it doesn't look right, it's because the camera wasn't parallel with the circle so the scribed lines would show up well.
Step 4: Layout Bolt Holes and Drill Them.
Now lay out your bolt hole circle and get to it!
In this case, I had a 1/4" hole in the center, and 3 equally spaced bolt holes about the center. (I lost one of the screws in the photo)
To lay out a 3 or 6 bolt hole circle, scribe the radius of your bolt hole circle. Using the radius of the bolt hole circle, draw an arc anywhere on the circle with the center of the arc also being on the circle. Now, the center of the next arc should be where the first arc meets the bolt hole circle radius. Repeat until there are six intersections.
If you need 2 or 4, use your scribed lines.
If you need 5, you are going to have to use a protractor.
I hope this helped you.