There is a sticky at the top of this forum by the Wonginator called "how to set up an egg tub". As to preheating, I usually stick my ready-to-go tub into the 'bator a day or 2 before I expect the lay.

My best advice - For substrate mix. You are better erring on the slightly too dry side than on the too wet side. You can 'fix' a dry egg fairly easily, wet eggs become the battle of the mold.

Good Luck, buddy.

Cheers,
Kat