Tile onto p b dont skimm it bring weight issues into tile fixing.
Tiling onto plasterboard youtube.
Dont use pva as the other guys have stated.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
As its a nice flat surface spread the adhesive onto the wall with a notched trowel then just apply the tiles with firm but steady pressure and use spacers i normally get a 2x1 batten and tack it to the wall 1 tile and 2 spacing a high from the work top then tile from that upwards when dry remove the batten and tile down to the worktop then grout but leave the last tile space to allow sealant in.
What size of tiles are you fixing to walls.
Any windows tile from centre of these.
You may tile over existing tile painted or unpainted drywall plaster and textured walls.
Preparation for tiling onto plasterboard plasterboard should be dry securely fixed and rigid with no protruding fixings with the face to receive decorative finishes exposed.
If you re not sure if you have lead paint test kits are available.
As for setting out centre your walls and walk tiles into corners to see what size of cuts you will have.
Tiling onto plasterboard walls.
However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight.
Let us know ansewers to the above 2 questions.