There is a special tool to do it, but it's not "needed". Both times I had to do my clutch, I used a small flat screwdriver and small hammer. I tapped the white ring progressively inward, working around it like you would tighten lugnuts.
The trick in my opinion is that white ring needs to go in farther than you think it can or should, to the point where you really can't see it anymore. That releases the fingers that clip the two ends together.