If it’s a latex-based paint, there is an easy way to do it without making a huge mess. You will need a garbage can and a water hose.

First, you will want to place the handle over the rim of the garbage can so the roller is hanging down inside of the garbage can. Next, spray water from the hose on the edge of the roller so the roller spins at a high speed. Move it up and down the roller until you can see that the water is clear. When you are finished, remove the roller from the frame and stand it on its end somewhere so it can dry.