Honestly, I think I just got lucky with bonding my bunnies.
I bonded Mochi and Milk together in the bathtub and did 5-10 minute dates few times a day. During the bonding sessions in the tub they would just ignore each other in the beginning then Milk would come up to Mochi to cuddle. Mochi was indifferent about Milk which was a good sign. They would eat pellets and hay together w/o nipping or biting. There was this one time that Mochi chased Milk around the tub, then I just separated them and tried again later on. I put them together in a pen when I felt comfortable and trust them to be together.
I tried to do a trio bond w/ Mochi, Milk, and Marshmallow and that didn't go very well because Mochi was too overprotective over Milk.
So that's why I got Penelope for Marshmallow. I brought Marshmallow w/ me when we were going to pick up Penelope. Put Penelope inside the carrier and they did fine. When we got home I just put Penelope back into Marshmallow's pen because I felt comfortable and I knew Marshmallow wouldn't fight Penelope because I knew he wanted a friend to be with. So they bonded instantly!
I fixed the boys around 4-5 months of age.
Few months of having their pens next to each other I took all of them to the park and put them together. At first they were indifferent, Mochi and Milk liked Marshmallow except Penelope (not spayed) they would growl at her. So I knew she had to get fixed first before trying to bond them again. So until Penelope was ready to get fixed, I would exchange their litter boxes every other day so they'd get used to each other's smell.
After Penelope was spayed, I decided to bond them around Thanksgiving break because I thought it'd take days to bond them, but it actually just took one day. I decided to take them for a car ride up a whinding road near our house, I had each pair in a separate carrier. While I was driving, my husband put Penelope and Marshmallow in Mochi and Milk's carrier and held it in his lap with the cover of the carrier removed so he could break up any fighting. I was driving for 10-15 minutes around the neighborhood. They didn't fight in the carrier or whatsoever. It's more of a "controlled stressed" environment that forces the buns to rely on each other during the car ride to cope with the sudden motion changes. Then when we got home I decided to place the pen in our living room where it's neutral territory. So when I put them in the pen they were all good, no fights, nips, or chase. All good signs! I was pretty surprised. Then I added the litter box and gave them vegetables for dinner. Left them their for 4-5 hours just to make sure they didn't fight. Then I put them back in the bedroom, I cleaned their pen, bought a bigger litter box. And they all lived happily ever after. Lucky me.
Then we got Munchkin. So brought all 4 w/ me when I was going to pick up Munchkin. Put Munchkin w/ the 4 buns and drove back home. Set up the pen in a neutral territory then put them together and the 4 buns accepted Munchkin right away. They were indifferent, they shared their food/litter box/water with him. I left them in the neutral territory for a few days until I put them back in my room.
And finally we got Léon. Brought the 5 buns to the Bunny Bunch and did a bunny date. Everyone was ignoring him except for Marshmallow, he wanted to hump Léon. After a few minutes being together and not fighting, put them all in the carrier and drove back home. This time I set up the pen in a different side of my bedroom (where they've been before but I cleaned it thoroughly). So the first night, everyone would leave Léon alone except for Marshmallow, whenever Léon got too close to him he would bite a few furs off, but no deep bites, fighting or chasing going on. It was going on for a few days until Marshmallow got used to him. I was wondering why Marshmallow was doing this to Léon and not to Munchkin, and I think because Léon wasn't a baby. Bunny Bunch told me Léon was only 8 weeks old, but I noticed his testicles were ready to descend. So Léon smelled really strong and he was already marking and spraying.
The IMPORTANT factor w/ bonding rabbits is to bond them in a neutral territory and have them spayed/neutered. Or get a baby because they're easier to bond w/ older rabbits, but it really depends on your rabbits personality. I just kind of knew that my buns wouldn't attack the baby. I read some posts online that some older buns would kill the babies. So know your rabbits well and be careful if you think they can be extremely territorial.
Also an important factor is to keep calm and don't get too nervous.
Before I bond my rabbits, I talk to them like I think they understand me ... I tell them there's going to be a new friend that's going to be living with us and be nice to him/her.
You could approach it more conservatively with the multiple bunny date method in neutral territory but it does take quite a bit more time and patience and usually after they're comfortable in neutral territory there's still some adjusting to familiar territory to see who will be dominant. I'm not saying that my bonding method is the absolute best or right way, this is just what worked for me several times. I am no expert in bonding rabbits. I have read online that bonding rabbits on car rides is the "stressful method" but when they can share a common experience together where they rely on each other for safety, it has helped them form relationships and watch out for each other as a group. After all, buns and people alike share that sense of family where we rely on each other for support and trust.
Good Luck!
