Move your furniture out of the way, perhaps to the sides of the room, or to another room if needed. Roll up your rug and move it out of the way also.
Vacuum or sweep the floor. Mop also, if needed, but make sure the floor is completely dry before placing your foam rug pad on the floor.
Lay your rolled-up foam rug pad on the floor. Double-check your rug pad's instructions to make sure your rug is right-side up. While some rubber rug pads are reversible, others have a distinct "floor side" and "rug side." Remove the wrapping, if applicable, then gently roll the pad across the floor until it is fully extended. Gently slide and move the rug pad until you have placed it exactly where you want it.
Roll your rug back in place, on top of the rug pad.
If the rug pad is larger than the rug and extends beyond it, use a pair of scissors to trim the pad. Ideally a thin rug pad should be about 1 inch shorter than the rug at all sides. For a thicker pad, cut it about 2 inches shorter. Cut very carefully so that you do not scratch your floor. Cut as straight as possible, but don't worry about making it perfect, since the rug will ultimately cover it up.
Move your furniture back in place.