Tiling should be set out from the centre of the walls either the centre of a tile or the joint between two tiles positioned in the centre of the wall whichever of these two options avoids small cuts of the tiles.
Sealing plaster walls before tiling.
When applying tile to a plastered surface it is important not to overload the weight of what the plaster will take before failure occurs plaster will support about 20kg m2 or about a 9mm thickness of.
To prepare your walls for tiling is rather simple as long as you have the right know how mindset patience and tools.
If you are using a cement based adhesive then you will need to prime the plaster with a suitable primer i personally only use weber pr360 it is a acrylic primer and i use this type of adhesive on all walls now apart from on kitchen walls with small tiles then i will use tubbed adhesive.
Before you get to it you should level the bathroom the kitchen and or the hallway walls.
You should not tile over wallpaper glossy surfaces lead paint or plywood.
However tiling over tile can add quite a bit of thickness so make sure your wall can handle the weight.
You could use a coat of thinned pva to seal the plaster or you could mist coat the plaster with a thinned coat of emulsion which will seal it and will not affect adhesion of the tiles.
If you re not sure if you have lead paint test kits are available.
Plaster is a weak material compared to brick and cement.
There are specific wall primers designed to take tile adhesive from bal etc but if you are on a budget then mist coating will be fine.
Make a mark on the wall with your starting point.
Make a gauge rod using a length of straight timber.
You can tile over a wide range of surfaces and textures but wall preparation is absolutely crucial.