There is many different scenarios for bunnies needing bonding. This post will give you a few ideas and ways in which you can bond your buns.
Baby bunnies....
When your buying baby bunnies and looking to get more than 1, get them from the same litter. I've been told that the bond between babies before hormones kick in is a false bond (not sure how true this is), however, I have always found it better this way as they already know each other well. TIP- buy a 2 tier hutch so that if you run into problems they can be separated and bought back together using the same hutch. FAQ- what genders should I get? Well... personally I would go with 2 girls or 2 boys if you want to be 100% baby free, however, girl and boy is the most natural pairing and tends to work better. You'll read a lot that 2 girls is best, I find two boys or 2 girls the same. Girls get territorial, boys get testosterone. Its 50/50 with both. I've had 2 girls together that hated eachother and 2 boys that got on great. Its the luck of the bunnies you choose more than anything else. Just keep in mind if you go boy and girl pairing that its not common but, its also not impossible for them to get pregnant before you get chance to get them neutered.
Keep them together if you can and there's no fighting, if you notice fighting, pushing each other away from food bowls or them doing what rabbits do best, split them ASAP. Get them both nuetered as soon as you can. NOTE- just because you see the rabbits doing what rabbits do best this doesn't mean that your 2 girls have suddenly become boy and girl. Rabbits also do this to show dominance (when breeding the girl can sometimes jump on the boy to say I'm the boss before the boy gets anywhere near her!)
If you've had to split your bunnies, try to still bring them together daily while your watching and they are in a natural place. As soon as you've had them neutered, and they have had a few days to recovered, start bringing them back together in the natural area while your watching. Then follow the advise for bonding older bunnies below.
Older bunnies...
When you buy two older bunnies or a new bunny to bond to your older bun, try to get them as similar age as possible. Make sure before you start the bonding process they are both neutered. Put them both in barred cages and places the cages next to each other so the bunnys can be next to each other, touch noses etc but can't fight. Leave them for a few days (you may notice nipping at each other through the bars, ignore this. They can't get to each other to hurt eachother). Then... leaving everything exactly how it is switch the bunnies around, so they are in the other bunnies home. They have the other bunnies bowl, water bottle, used bedding and litter tray (if using one). Leave them a few days and repeat this process. Then take the bunnies out of their cages and place them in a neutral space. TIP- put 1 bunny on a bunny lead, if it turns nasty you can pull the lead back and away from the other bunny without having to get your hands in the middle of a fight. Rabbit bites can be nasty! If the bunnies get on fine and there's no nastiness at all, allow them to keep meeting daily for a few days. (If theres fighting. Go back to the swapping cages and try again, if they fight the next time round id cut your losses.)
Once your happy they are fine and you can leave them together outside their living areas...
Personally I'd get a brand new home, if you can't do this, make sure the sleeping area and living space is completely cleaned and Milton sterilised. All new clean toys, bowls, bottles, bedding. EVERYTHING MUST BE FULLY CLEAN WITH NO BUNNY SMELL OR NEW. Now allow both bunnies to share this space as their new bonded home.
WARNING- Keep some TCP in during bonding. Alway check all bunnies over thoroughly after together time. If theres any bite or scratches treat them straight away, if infection sets in it can lead to an abcess! Use tweezers to remove dirt, claws, fur etc and bathe in very watered down tcp. Do this daily until wound has closed. Sudocrem can also be applied if clean to encourage the skin to knit together faster.
Hope this helps everyone. If anyone has any other bonding tips or things they have heard or used please add these into the comments below.