Hi,I'm in UK and unlike Mike's friend,don't have a greenhouse.However indoor winter grafting isn't climate dependant.As long as it's above 15 degrees it will work with this method.Keep out of direct sunlight.
Bring the rootstocks indoors a couple of weeks or so before grafting.Try and graft as close to the time when you usually expect your maples to wake up...for me end of Jan is just a shade too early,early Feb is better here.They usually take about 3weeks to take.You will not be watering them again until the graft has taken so make sure they're lightly moist not dessicated.
After the first week cut the tops of rootstocks and let them bleed,then a few days later cut back more,you're trying to get the bleeding out of the way before you graft.Once they've stopped bleeding(if they bleed,sometimes they don't)you're ready to graft.If no bleeding,just look for other signs the rootstocks are awake(swelling buds,new roots)the roostocks must be awake,it's better to wait longer to make sure rather than graft on a dormant rootstock and have them fail.
After grafting,individually put a small bag over the scion only(don't cover the rootstock) and tie the bag at the base of scion with a twist tie.I find cutting an inch slit at the bottom of the bag(open end)helps...you can then slide the bag down the scion with the rootstock positioned in the slit and tie the bag to the rootstock. (I use little freezer food/sandwich bags but you may be able to source better)This is a must do as they will dry out otherwise.A squirt of fungicide into the bag is recommended too,fungus is the main killer of my grafts so keep a daily check on them.Humidity is not an issue as the bag takes care of this.
After about 3weeks you should see some action,the buds will swell then leaf out.If you see blackening at the base of the scion it has failed.Keep the bags on the scions while the leaves grow.Once the scion is leafed out properly and looking happy and possibly getting cramped in those bags,you can now remove them and put a bag over the whole plant instead and elastic band round the pot.Just don't remove the scion bags too early as the leaves will quickly shrivel.You have to be the judge of this.You can also give them a sip of water if needed but only a little.Now you will have to ween them off the humidity over the next couple of months as the grafts get stronger.I'm sure you can think of ways of letting more air into the bags gradually,just don't be tempted to remove their protective environment completely too early,especially in your centrally heated house.Don't rush it,they'll be quite happy in their little 'greenhouses'.
I've had over 90% success with this method but sadly(and most annoyingly)I just don't have the same success with summer grafting.....good luck :)