Hi there, welcome!
Congrats on the new bun, and thank you for adopting!
The usual plan is to put them in cages a few inches apart right away, but then give the new bun at least a week or two to settle in before doing any bonding or pre-bonding. If the new bun seems really scared, give him more time.
Then you can start swapping who is in what cage every day or two to let them get used to each other’s scents. You can do this for a week or more (some recommend 4 weeks minimum), but really it comes down to how the buns behave towards each other and how they react when you swap cages. You might notice when you first start swapping cages, the rabbits will run around a lot, chin everything, mark all over outside the litter box, etc. They might run the fence and try to “get at” the other bunny. Then after a while, they will barely seem to notice they are in a new territory. They will not mark as much, they won’t run the fence, they might even lay near the other rabbit on the opposite side. You also will notice their behaviors start to sync up (they will groom at the same time, eat hay at the same time, etc.). Once you see a clear shift from territorial craziness to relaxed behaviors, you are usually good to start sessions. Never hurts to add in a bit more pre-bonding though if you aren’t sure.
Continue the swapping once you start actual bonding sessions in neutral space. You didn’t ask about this, but once you do start sessions, find the most neutral space possible to work with them in. 
.
.
.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.