To introduce a new bunny, we recommend the following steps:
With your new bunny watching, snuggle your present bunny on your knee then put him/her down to watch while you then snuggle your new bunny. Now snuggle both bunnies together on your knee and offer lots of nose-rubs and shoulder massages - whatever seems most popular and appreciated. Sometimes, the two bunnies will bond in this way and stretch out together to enjoy more fuss.
Please find a neutral area which your present bunny/ies have never been in such as an area of your garden, a garage, garden shed, bathroom, the bath, the back of your car .... anywhere which is unfamiliar and won't carry the scent of either bunny. Now, follow the snuggling-routine as above.
Next, pop both bunnies down on the ground/floor together and watch. They will probably sniff at each other and your present bunny might lunge at the new arrival. If so, push his/her nose to the ground/floor in a submissive posture and fuss both of them together. If your present bunny is a boy, he will probably mount a girl which is normal, so just leave him, assuming he has been castrated, isn't actually penetrating her or she isn't becoming distressed.
Let them explore together. A little chasing is normal as they establish who is to be the dominant partner/pal. Let them do this or even pull a little fur. If there is fighting with biting, separate them immediately and try again later. Spraying the mis-behaving bunny lightly with a water bottle can help.
Once they're getting on together in the 'neutral area', offer a little more space and more snuggles together.
Then, pop them together in their new home. This must have been thoroughly cleaned to remove as many traces of its previous occupant's scent and so smell as neutral as possible. Try to alter its position in the home or garden so that it becomes a 'new' cage/hutch/bed and its former occupant will be distracted a little by exploring his/her new surroundings.
Bonding can take patience with some rabbits but is always worthwhile and it's very rewarding and enjoyable to watch your bunnies snuggle contentedly together.
Often, the bonding is 'instant' and it isn't actually necessary to follow the tips given above, and your bunny will be much happier to have some bunny-company.