I'll reply, but I'm somewhat of a n00b at using heated undergarments.
Regarding layering, my electric undergarment is a
Light Monkey heater base. It's essentially an undershirt, so I use it as a bottom layer and then put my fluffy layer over top.
Regarding when to run the heat, that decision depends on a few factors.
One is your battery. If it's huge, then you've got the option to stay warm all day long. If it's not giganto-normous, then you'll need to ration the power.
The other -- and probably far more important -- factor is what kind of dive you're doing. If you're doing a recreational dive and staying far away from no-deco limits, then you can probably run the heat as long as your battery allows. If you're pressing no-deco limits or doing deco diving, though, you probably want to save the heat for the ascent profile. If you're toasty-warm on the bottom, you'll ongas more efficiently, and that could mean decompression problems. If you're cool on the bottom and warm on the way up, you'll offgas more efficiently.
That being said, I think there's no good reason to freeze yourself on the bottom. In my case, I usually do 1-2 hour runtime deco dives in 50-F water, and I have a fairly wimpy battery. If I get cold on the bottom, I'll turn switch the power on for a short while, drive away the chill, and then switch off the power. Then, on deco, I'll heat myself up until I'm comfortable. Using heat only intermittently at depth saves battery, keeps me from feeling miserable, and doesn't appreciably run up my risk of DCS.
