Teach them some about some of things you do to prepare for games. Run it like a little workshop. There are a number of things you do to prepare ahead of time, or things you do during a game. Isolate those things and then teach them how to do them. Here are a few examples:
Things like how to plan "random encounters". For me, I determine what might be encountered in a given area (according to where the party is heading) and then do little write ups, on index cards or such for what might be encountered (plus a few write ups for things in different directions). Then when an encounter is needed, I select one that appropriate. Thus the actual random part is when and where the encounter is, not what it is. And I include just "sighting" a monster as an encounter as well. So one encounter might be seeing a huge dragon flying across the sky some distance away.
Teach them about how to bring a party together (always one of the toughest things for me).
Teach them how to run a combat. Set up a combat scenerio and run through it with them. Give them tips about what you are doing and why. Then have each of them run the same or maybe different combat encounters. With either you or your wife as a player, and the other of you working with the fledgling GM (i.e. sitting behind him and giving pointers, not actually running it). Do this a few times, with the "helper" GM being less and less active each time until they get the hang of it.
Teach them good GMing habits such as
- How to avoid railroading players
- How to adapt when players do not follow any plots set up for them
- How to quickly adjudicate a situation not covered by the rules
- How to keep the game moving
- About the importance of always giving PCs a chance (i.e. no killing them off-screen)
- How to determine how much treasure to give out
- How to determine ELs and CRs and how to use them effectively
- The importance of not having PCs being overshadowed by NPCs
In the GM section of HARP, there is a small list of general tips for newbie GMs. Draw up something similar for your "trainees". Give them concrete goals and such to work towards. Remember to not focus just on the good things, but also the bad things and how to avoid doing them.
And remember, they may not catch on right away, and it may take them some time to fully integrate the lessons you teach them. Not everybody will learn at the same rate, to make sure to be patient.