How do I tile over floorboard. Does flexbond work??

I would just suggest that you lay the thinnest plywood down before tiling

The normal product used is call "thin set". IT normally comes as a powder that you mix with water (like cement mix) or as a more expensive pre-mixed. Thin set should be mixed to peanut butter consistency. An electric drill with a paddle mixer works well. Put a glob of thin set on the floor and spread it with a trowel, then use the notched edge of the trowel to make ridges of thin set. The tile is then laid onto the thin set, spaced, and leveled. Work your way out of the room, and let it sit overnight before walking on it and adding grout.