When I'm training a new ASM, I like to assign a specific area to her/him for rehearsals, so they feel like they have some ownership without getting overwhelmed. I usually assign costumes, props, or shift choreography, and explain or give a checklist for whatever area I've assigned. I check in with the ASM at every break to see how they're getting along, and if they're bored, I step up my expectations in terms of paperwork or perfection, and if they're overwhelmed, help them to prioritize and organize. By the time we get to tech, the ASM feels like they at least know all of something, which can mean a lot when you're new to the job. A day or two before tech, I sit down with the ASM and go through the entire show from preset to post-show scene by scene and make sure we both know exactly what needs to be done, and which of us is doing what. Then it's a matter of constantly communicating with the ASM during tech, letting him/her know exactly what you expect and where you expect them to be when, and that they can call you at any point if they are unsure. One thing that is extremely important when training a new ASM is that you must present to the cast and crew a united front, that you and the ASM are a team, and that the ASM has your complete trust and confidence. The new ASM will not necessarily be able yet to command respect from the cast, and will appreciate and (hopefully) live up to your vote. If anything goes wrong, DO NOT reprimand the ASM in front of cast and crew. Deal with them diplomatically and privately, as you are a team. They will appreciate this as well, and will repect you the more for it.
Good luck!