Search down the main board until you come to these posts (Ctrl F and Concrete will get you there)
Beyond that, yes, compact the dirt (you can rent a compactor, but a 10lb sledge would work okay to tamp it down too.
Lay about an inch of 1/2" or smaller gravel in the hole and pour away. For the type of work you are doing, you owe it to yourself to get bids from professional flatwork contractors before you get into this. The pros are good at acheiving a FLAT floor with a SMOOTH finish, instead of a "homeowner special". Weigh the cost of materials, renting a mixer, hauling 60-80 lb sacks of premix into your basement, hoping you do a good job, and the general pain in the neck of the process, versus the pain of getting out a pen and writing a check. (I used to change my oil on my cars, but why bother? I'd rather spend the $20 and let someone else get greasy)
I am a great proponent of DIY, but not always. I think this one's better for a pro.