Although they are social animals, rabbits are also very territorial so be prepared for a stressful time and plenty of fighting when you introduce a new rabbit, as your existing rabbit will initially resent the invasion of its territory and fight to protect its turf and dominance.
The most important factor is to introduce the rabbits when you have the time to supervise them closely. To start, you need to introduce the rabbits on neutral ground, that is, not on your existing rabbit’s turf. One suggestion is that you take your rabbit with you to pick up the new rabbit and have them meet in the car or at someone else’s house. At home the rabbits should be housed separately, but within eyesight of each other. Then each day you can let the rabbits out together in a neutral space (a room or area your rabbit doesn’t normally go into) for 20 minutes or so, but make sure the supervise them during this time. The area should be quite small and safe so the rabbits can’t hide or escape.
Keep a water spray bottle with you to break up any serious fights or watch for imminent aggression and prevent a fight from occurring. If they are fighting, then continue housing them separately (but within eye contact). If they are not fighting, then you can attempt putting them in the same cage together but only when you are there to supervise and remove one if necessary. If the existing rabbit’s cage is large enough, then it may be satisfactory to house both rabbits, but it must be thoroughly cleaned of the existing rabbit’s scent. Rabbits rely on smell for communication more than sight, so if possible, consider getting a new, larger cage to house both of them helps prevent territorial fighting.
Once housed together, it is still a good idea to keep the rabbits separated when you are away until you can be sure no fighting will occur. This process can take a long time, weeks to months, so you need to be patient and spend time working with the rabbits and gradually bringing them together from neutral ground to normal territory.