Remove the butt-cap
First of all, you need to take out the butt from the rear of the pool cue. If the pool cue does not have any removable parts, you cannot modify its weight and length. However, most new, high quality cues are adjustable, and you should confirm this at the time of purchase.
Use the Allen key
Insert the Allen key in the cue and turn it anticlockwise to remove the existing weight in your cue. Some cues do not have a place for the Allen key to be inserted. In this scenario, you should consult the cue's manual for instructions on how to remove the weight.
Insert the new weight
Turn the new weight in a clockwise direction in to the butt of the cue. It can be easily tightened with your hands, without using any kind of tools. However, it is recommended that you use the Allen key to finish inserting the weight in the cue. Having softly tightening the cue, you need to stop when there is obvious resistance. Make sure that you do not try to tighten the cue too much; otherwise, you may end up damaging your cue or its balance.
Reinsert the butt cap
In order to finish changing the weight of your pool cue, you should insert the butt cap on to the end of the pool cue once again.