Step 1: Cutting the circle
Step 2: Start cutting
I started cutting the 4 angles to have an octagon. I found this cuts to be the most difficult as the saw tends to move the wood but holding it firmly against the sled witha push pad made it easier.
Step 3: Turning and cutting
Turning the MDF a bit every time I kept cutting the points until it had no more points and looked like a circle.