Used to be with side reins on a roller, stood in for some time, with a bit with keys to encourage them to play with or mouth it. These days, once they are happy to accept the bit, I would rather do it in hand, asking for relaxation and giving when they offer it
i just put a bridle on with no noseband and a key bit and left her in it for a short while each day maybe 10 mins while i was grooming her, i never left her without supervision with the bit in..., i then used a rubber bit when i showed her in hand and took her for walks. seemed to work ok...