STEP ONE: Procure manure. Important here: use ONLY fresh horse manure mixed with sawdust/pine shavings bedding, or straw or hay. Using other manures sometimes can kill your seedlings if there is too much off gassing ammonia. It's happened to us, don't let it happen to you!
Some things to consider: in a few weeks, the heat will die down and you may want to extend your hot bed further. Turning the bed is complicated, so I would just recommend making another bed.
In the late spring when you want to plant your high tunnel, just tear down your hot bed, and the manure should be composted enough to work into your soil! Make sure that you do supplement with nitrogen because the compost won't be totally broken down.
Rats are a big problem, because they like a) living in giant, warm saw-dust filled beds in the winter, and b) they like to eat seedlings of very expensive plants, like broccoli and peppers. I give you permission to trap them with any means necessary.
If you have questions, e-mail us at plowbreakfarm.com.