I think one of the most common ways of doing this is with a Newspaper combine...did my first just the otherr day, and it apperars to have went well.
Take a single sheet of newspaper, using your hive tool or a razor knife, cut two to three decent size slits in it. The slits will give the bees a starting point. Take the cover off of hive A and lay the sheet of newspaper on the top, covering the entire box. Take hive B and set it on top of the newspaper. The bees from both hives will chew through the newspaper and remove it from the hive in the next day or two. This will give them times for their pheromones to merge and for them to become familar with each other. Be sure to provide a top entrance or crack your top cover to allow the bees in the top box a way in and out until they are done with the paper. If not, and it gets too warm, the heat may kill them.
As with all things Beek, there are differing opinions on how much newspaper, how many if any slits, and whether or not you leave the paper dry or spritz it with sugar water.
However, the above method seemed to work fine for me. I was amazed at how quickly the little piles of shreeded newspaper started appearing in fromt of the hive.
Good luck! :)