The first thing to train them on is to "come/here" to you. As for field training: If they are pups don't train them together until each has started running rabbits on its own. Take the pup to an thicket area where there are lots of rabbits and just stand there and let the pup nose around and explore. When the pup loses interest and wants to come back and jump up on you, start moving again real slow until it starts to nose around again, then stand and wait.
Most beagle pups will start to trail and sound off on a rabbit at 6-8 months of age if they are given plenty of opportunities to get in the field.