The carpet will go in a finished basement. The basement has bare concrete right now.
There is little that you have to do. Just ensure that the concrete surface is entirely dry.
There probably won't be a preparation other than cleaning, but you need to be sure the floor is pretty dry and likely to stay that way. If you have never seen water or dampness on the floor- good. If you have seen dampness, you have a potential for mildew. Be sure the carpet you buy is all synthetic. If there are any natural fibers in it, the materials will hold water and promote rot. Most carpets of today are all synthetic. The tack strip is nailed directly to the concrete. Pad is taped into place, and a few people use a string of adhesive to help hold it in position. The carpet is installed just as it would be upstairs on wood, but the labor is a bit higher. Some carpets are glued down, all that requires is a moisture proof adhesive; again- fairly normal.
If your basement is at most, a little damp, then I'd hesitate before putting in carpeting. You don't want mold to form under the carpet. Try to dry the basement out first. Maybe all you need is a good dehumidifier that you will continue to run. If you're not sure about the floor, do a simple test. Tape a 3x3 foot plastic sheet onto the floor. Seal all edges. Come back in a few days. If there is moisture between the floor and plastic, don't finish the basement floor with carpet. Once the basement is dry, just put an 8-pound pad under the carpet. You don't need roofing felt under it. If you are gluing carpet or carpet squares directly to the floor, be sure to seal any and all cracks in the cement first. Also, use a water-soluble glue in the event that you have to rip up the carpet due to a flooded basement. Don't cover the drains with carpet or, at least, mark them off so you can find them later. There is a spray mold-retardant that can be applied to the carpet after it is laid down. We had that done in our basement. I'm not sure if that can be done prior to laying the carpet, i.e., to spray the floor and/or the padding first.
