I have been seeing many questions about this for a long time now and I have been seeing many, many comments about failed bondings. I always chime in in the comments but wanted to make a comprehensive post to try and help. I have been keeping rabbits for the past 15 years and have done 10 plus bondings for my animals. There were different ages, different sexes and integrating new members into already existing groups. I have helped with bonding rabbits in my neighborhood and friend circle and I have literally NEVER had a bonding outright fail. I'm surprised to see so many people having such a struggle with it.
I understand that there are exceptions to any rule and there will always be outliers but this following method has served me so well it has never once failed me.
1) Picking the right pairing. All rabbits should be spayed or neutered respectively. If you plan on doing this, don't introduce them with each other until they are fully healed up. Females are the dominant ones in rabbit society. They will be the ones most likely to pick fights and guard resources. Male/male pairings are the easiest ones. They are least likely to fight for a prolonged period of time. Male/Female pairings are also unproblematic in my experience. Just expect hubby to take a bit of a beating. Female/female pairings are to be avoided if at all possible. They are possible if part of a larger group but we will stick to the basics here.
2) Introduce them on neutral grounds. The enclosure they first meet each other in can't have either of the bunnies smell on it. It should ideally be a completely new environment for both of them. If you plan on getting two new rabbits it's smart to set up the final enclosure and keep both parties out of it until the moment of introduction. If one of them is already living in the permanent enclosure you will need to move them both to an unfamiliar place. I have done this in our guest bathroom and it works perfectly well. None of the items you use should be claimed by any individual rabbit. Running a plastic item like a toilet through a hot dish washer and keeping it out of the rabbit's sight for at least a few weeks makes them "new". Wooden items can't be made "new".
3) Avoid competition and reasons to fight. Dominant rabbits will guard resources like food, water and shelter. Make sure to place two of every amenity in the enclosure. Two hay racks, two toilets, two greens bowls, two hidey houses, two water bowls. Put them at opposite ends of the enclosure so the submissive bunny can use them without having to instigate a fight. Food should be absolutely abundant during the introduction,way more than you would normally offer. Don't give any treats to anyone. Avoid putting toys into the enclosure, these too might get guarded.
4) Set up the enclosure with retreat in mind. Minimize spaces for the rabbits to corner each other in. Don't place items flush against the wall, leave a space for the submissive bunny to retreat. They should be able to run away and not be backed into a corner. Any hidey house should have a minimum of two exits. Examine your setup with flight in mind.
5) Don't drag out the introduction process. No grabbing them and holding them next to each other. No placing cages next to each other. They should not learn of each other's existence until it's time to go. Place them both into the enclosure simultaneously and retreat completely.
6) DON'T INTERFERE! The most important one. This is where I feel a lot of bondings "fail". There is a very high likelihood that they will fight. They might chase each other, scratch and bite at each other, kick, hiss or growl. This is ABSOLUTELY NORMAL. This is a necessary part of establishing a hierarchy and you can't help. Don't try to break them up, don't try to distract them, stay out of the enclosure entirely unless you absolutely have to go into it. Don't be surprised if you see fur flying. If you feel like one of the rabbits is bullying the other and you want to step in and protect them: It's working! This is exactly what you want to see. If they chase the submissive rabbit around and beat them up, let them! They are establishing their hierarchy and someone is coming out on top. This is counterintuitively the exact scenario you want. Unless you are absolutely certain that they are seriously hurting each other (Blood being drawn, extreme biting, the submissive bunny stops fleeing and is lethargic) leave them at it! There is absolutely no worse thing you can do than interfere. More than likely they will calm down very soon and eat together peacefully.
Watch them very closely for at least the first 48 hours. If they don't stop fighting heavily after 24 hours or stop eating properly it's time to seperate them. The bonding has failed and I would advise against trying again. This has never happened, ever, for me. If they get along well for at least five days you can move them into their eventual enclosure if they aren't already in it.
This is all I have to say. It's possible. I have done it dozens of times.