Use a proper acrylic based tiling primer sucha s bal prime apd or ardex p51.
Sealing plaster walls before tiling.
This way the adhesive will stick properly and last in the long run if you are using a good product.
You didn t say what adhesive you are going to use but one thing that is absolutely definate do not use pva.
Pva is not suitable for tiling for various reasons and this has been debated many times here in the forum.
Thoroughly vacuum the area and give the wall a once over with a damp sponge or cleaning cloth.
Also now it s been plastered you will be restricting yourself to a tile with a max weight of about 16 kg m2.
If you choose a heavier tile you will need to overboard with a backaboard.
Sand with a medium grit sandpaper to create a uniform surface that s not too smooth.
If in doubt prime the walls is a general rule.
He gave me a list of their recommendations and one of them was to always use waterproof pva diluted before tiling on new plaster.
Otherwise a few complications might occur.
Also keep the walls you are about to tile free of moisture dust and grease.
Don t use paint glue pva wall paper paste only use the primer recommended by the adhesive manufacturer of your chosen tile adhesive.
Adhesive grout and tile 20kg max weight onto plaster.
Seal the walls with a skim coat and let it dry thoroughly.
New plaster work should be fully dried before tiling work is commenced this drying time varies upon temperatures but can take up to 4 weeks before fully cured.
Before you get to it you should level the bathroom the kitchen and or the hallway walls.