I recently did this on my floor, which was concrete and I used a self-levelling floor underlay product. You can go to any home improvement store and ask for this. They have a quick dry formula also. I used DuRock© Self-Leveling Floor Underlay but I'm sure there are others on the market too. It is non-shrink, fast setting and gives a smooth surface over wood and concrete which can be up to 3" thick.