Ceramic tile needs a firm, solid surface under it that won't soften with moisture or move over time. Timber floors aren't a good base for ceramic, especially in a bathroom or kitchen where heat and moisture can cause it to expand and contract under the tile. Instead, cement board is the preferred underlayment for ceramic. It looks and feels like drywall, but once installed is like a layer of cement.
Go over the wood floor with a belt sander to remove all dirt, gloss and other surface material. Clean up the dust.
Use household cleanser to get the floor completely clean. Let it dry for at least a full day.
Set a sheet of cement board in one corner of the floor. Use a pencil to trace around the edge of it. Lift it out. Spread thinset mortar over the area with a notched trowel within the pencil lines.
Set the sheet back in place, pressing it into the mortar. Shoot galvanised screws straight down through the surface every 6 to 8 inches.
Set the next sheet of cement board in place off the end of the first one. Trace around it, spread mortar over the area, press it back in and screw it down.
Repeat and continue, laying the boards end to end. Cut the final board of the row as needed with a jigsaw.
Start the second course at the opposite end of the room as the first (so there are no four-way intersections). Cover the whole floor. Let the mortar set for a day
Press drywall tape over the seams of the boards. Cover them with mortar, using a drywall knife to flatten it. Let it set for a day.