Short answer? What bobbi-j & Kikisgirls said.

Put them in the coop with a heating pad/plate.
I raised 8 chicks hatched March 26 in a kiddie pool in our garage with a heat lamp.
They outgrew the pool in 2 weeks and my husband built me a 4’ x 5’ x 2’ high grow out pen we also kept in the garage.
We had to put a wire “lid” on it shortly after because they could easily jump out.
They outgrew that by 6 weeks I’d say.
The 12 chicks I have now (4-5 weeks) were started in the kiddie pool for 2 weeks and then transferred to my prefab coop.
This time I used a Premier 1 heat plate and I love it!!
I’ll never go back to heat lamps.
We ran an extension cord out to the coop and the heat plates cord fit perfectly right under the nest box lid.
They love their coop and run!
I make sure to give them little roosts and things to jump on.
They mastered the ramp in no time.
They’ve been going up at dark by themselves for about a week.
They have a good time in there.
They’ve been off of heat for about 10 days now.
I only had it turned on at night anyway.