Living near each other for a bit, swapping cages so they get used to each other's smell, neutral territory, a distraction (food - I use other bunnies, but not always a good idea - mine are used to other rabbits), places they can run & hide to (I use quite a large place to bond BUT, I know some people use a smaller space so buns HAVE to be near each other), a car journey.
Mostly when I've bonded, there's been a couple of brief introductions, then I 've just left them together (obviously wouldn't if I saw blood!)