Sounds like you're doing great!

For humidity: other than misting and soaking, you can also add a moist hide (damp moss or paper towels in a hide) and just rewet it when it dries out.

For substrate: I tried the bed-a-beast, cypress and jungle earth (I think). The first was dirty and molded, and the others didn't make much difference for humidity.

I use aspen and prefer to soak personally.

Some snakes I can put in the tub with a wet cloth over top of them and they'll stay there. I can't handle them first though; they don't stay if they're "woken up". Make sure there are no holes, etc. for him to get away if he gets out of the tub. And don't forget him, he'll cool down fast out on the floor. Make sure the door is closed securely. Warn your roommates that the snake is taking a bath.

For the smaller/more active ones, I use tupperware containers. I don't worry too much about air, I figure that there's plenty in there for an hour. I don't worry about drowning either, but both are choices you have to make for your own conscience. I've heard to not fill up the water more than half way up the snake's side. Also don't use water that feels warm to you, it will be much warmer to the snake. Use a thermometer if you want. I think 80 would be good but that's a guess.

Have fun!

Oh also, soaking is great right up to shed time, and most crucial in the 48 hours *after* the eyes clear. If I even soak once in this time period, all is well, but earlier you may have to do it 2-3 times.

Some snakes will have a crappy shed the first time due to stress from acclimating to a new home (my experience), so don't doubt yourself if it's not perfect this once.

Oh and look to make sure the eyecaps and tail tip come off in the shed.