Once the wires are attached, you can gently fold any excess wire and compress the dimmer into the electrical box. Move slowly to ensure the wires don’t loosen or break if you have to force the switch into position. Tighten the screws to secure the switch to the box, and fit the switch plate. You will find that tightening or loosening the switch screws in the box by a few small twists makes a difference with how well the plate fits over the box. Once the cover is in place, turn the breaker or fuse back on, and check your handiwork!
Not all light bulbs are intended for use with dimmer switches – the wrong combination can damage the bulb as well as the dimmer electronics. The bulb's packaging will usually indicate if it is dimmable.