What preparation do I need to do before laying a slate tiled floor?

Hi The floor should already have a damp course, no need for one before tiling. Ensure the floor is clean and dust free, prime the floor use an SBR Primer not PVA. Use a flexible adhesive and tile away. If they are slate they will need a sealer on them before grouting or you can seal them before you lay them. If the tiles are various thicknesses ( not unusual with slate) save the thinner ones for the edges for cutting ease. Tile up to the edges leaving a grout gap if desired or if there are skirting boards consider removing them and replacing after tiling. Good luck Steve