Tongue-and-groove flooring isn't the ideal underlayment for floor tiles, because the wood of the floor can be loose, can move over time, and will soak up any moisture that gets under the tiles and potentially soften, causing cracked tiles. You're always better off installing a layer of cement board. If your circumstances don't allow that, secure and seal the floor before tiling it.
Walk across the wood floor in systematic courses, looking for any movement in the planks. Wherever you find any movement, drive in one or two drywall screws to secure it, using a screw gun. Get the screw heads completely under the surface of the wood.
Slowly pour floor levelling compound over all gaps or holes in the floor to get them level with the rest of the surface. Let the levelling compound set overnight.
Roll out a coat of sealing primer over the whole floor, using a paint roller. Let it dry overnight.
Snap two intersecting lines over the floor with your chalk snapline, so the surface is divided into four squares.
Apply tile mortar over the middle of the floor, covering several square feet. Press the first tile in place at the intersection, bordered by the lines on two sides.
Lay additional tiles around the first one, putting spacers between them. Spread more mortar and lay more tiles, building out to the edges of the room. Use a tile cutter to cut the tiles around the perimeter before you lay them, as needed. Cover the whole floor. Let the tile mortar set overnight and pull out the spacers.
Apply grout to the floor, pressing the grout into the spaces with a rubber grout trowel. Use a dampened sponge to wipe up the excess grout. Let the grout set for two days before walking on it.