LOL, not an expert (I just used google

), but yes I'm trying to breed trees....
The beehive doesn't have to be between the trees, just within a certain area (I don't know how many blocks away the bees will travel, but you're probably safe at least 6 blocks away). My setup I have about 6 beehives in the centre of a ring of trees...... previously I had set the trees out in a grid with beehives scattered around. I don't know which is a better way of doing it..... but I guess the important thing is you need the leaves, so plant the trees far enough apart so you don't have any of the leaves touching, because you need as many leaves as possible.
As far as the bees go, any bees seem to work, however a bit of a pro-tip for you is that rocky bees seem to work in the rain and at night... so if you can use rocky bees then your bees are working constantly for you and you might get there quicker. Whereas most of the other bees I've used are dormant in the rain or at night.
So you need to plant 2 or more types of trees, and the bees will eventually (it can take a few days) cause some of the leaves of the trees to "mutate" (that is turn a different colour).
A "mutated leaf":
If you break those mutated leaf blocks, you have a chance of getting a sapling (just as normally you can break some leaves and randomly get a sapling or not).... Those saplings should be a new tree breed. So then you plant that new sapling with different trees and continue on.
Some trees may need to be planted in specific biomes to work though.
So far I've bred birch and oak trees to get silver lime trees, and I've read that you need to breed the silver lime with a jungle tree to get a teak tree.... but so far my jungle and lime trees are giving me a different kind of jungle tree (has leaves that look almost like they have fruit on them) and a different kind of lime tree (grows taller)...
So given you get such a small chance of getting a sapling from a leaf block, and I'm not getting the saplings I need.... it is quite frustrating. I've tried googling again to see if that guide I was following was wrong and I need different trees..... but I'm finding it hard to get any other info
So here's the cheatsheet I found if anyone wants spoilers:
Tree Breeding (Spoilers) | Feed the Beast