The batten does the work, and going without it is the usual mistake
Almost nobody tiles a wall starting at the floor, and the reason is that floors are not level. Lay the first row directly onto one and the whole grid tilts by however much the floor falls across the room. By the time you are five rows up the drift is obvious against a window or a ceiling.
Instead you hang the tiling from a straight line you put there yourself.
Find the lowest point of the floor, measure one full tile up from it, and screw a straight batten across the wall on that line, checked with a level rather than measured off the floor. Tile upwards from the batten. When the adhesive has set, take it off and cut the bottom row individually to follow the floor.
That bottom row is a series of slightly different cuts, and it is the least noticeable place on the whole wall for them to be.
Comb it on, and never use dabs
Adhesive goes on with a notched trowel sized for the tile, combed in one direction across the wall, then the tile is pressed in with a small twist so the ridges collapse into a continuous bed.
Five dabs, one at each corner and one in the middle, is quicker and it is the reason tiles crack. A dabbed tile is a sheet of ceramic bridging voids, so any knock over a void breaks it. In a shower it is worse: grout is not waterproof, so water that gets past it collects in those voids with nothing to dry it out.
Large format tiles usually want the back buttering as well as the wall combed, because they need more bed and are far less forgiving of a wall that is not flat.
Wet areas need more than moisture-resistant board
Inside a shower enclosure, tiling straight onto plasterboard is the expensive failure, and it fails slowly enough that nobody connects it to the original job. Even moisture-resistant board is gypsum faced with paper, and grout lets water through.
A cement backer board, or a proper tanking system applied before any adhesive, is what belongs there. Outside the wet zone, sound plasterboard is fine.
Grout the wall. Silicone the corners.
Wait for the adhesive to cure properly before grouting, typically at least a day and longer in a cold room. Grouting into soft adhesive traps moisture behind a sealed face and lets tiles shift while the joints are setting.
Then the rule that catches nearly everybody: every internal corner gets flexible silicone rather than grout. Where two walls meet, and where tiling meets a bath, a tray or a worktop, the two surfaces move independently as the building and the fittings expand, settle and get loaded. Grout is rigid, so it cracks along the line within a year and then quietly lets water behind.
Grout the flat field. Silicone every angle. It is a five minute decision that decides whether the bathroom looks tired in eighteen months.
Counting the tiles, and the spares that matter later
Work out the quantity before any of this, because where the cuts land is decided at the same time as how many you need. The tile calculator covers that, including why a bare area figure is not a number you can order from, the two checks worth doing before anything is fixed, and how much adhesive and grout goes with it.
Keep the spares from the same batch, properly stored. Tile colour varies between production runs, so a replacement bought two years later rarely matches, and a single obviously different tile is more visible than a chip. Room area gives the measurements to start from.
Common questions
Where do I start tiling from?
Not the floor, which is the instinct and the mistake. Floors are rarely level, so a bottom row laid straight onto one drifts out of level as it crosses the room and every row above inherits it. Instead find the lowest point of the floor, measure one full tile up from there, and fix a level batten across the wall at that line. Tile upwards off the batten, let the adhesive set, then remove it and cut the bottom row to fit the floor last.
Should I use dabs of adhesive or spread it?
Spread it, always, with a notched trowel sized for the tile. Dabs leave voids behind the tile, and a void means the tile is unsupported, so anything striking that spot cracks it. In a shower it is worse than that: the void holds water that gets through the grout and it has nowhere to go and nothing to dry it. Comb the adhesive in one direction and press the tile in with a slight twist so the ridges collapse into a solid bed.
Can I tile onto plasterboard in a shower?
Not without protecting it, and this is where bathrooms fail expensively. Even moisture-resistant board is gypsum with paper on it, and grout is not waterproof, so water passing the grout reaches the board and softens it over years. Inside a shower or a wet area the right answer is a cement backer board or a properly tanked system, applied before any adhesive. Outside the wet zone, sound plasterboard is fine.
How long before I can grout?
Whatever the adhesive says, and typically at least 24 hours, longer in a cold room or on a non-porous surface. Grouting too early traps solvent and moisture behind a sealed face, so the adhesive cures slowly or unevenly, and tiles can move under the grout and crack the joints later. It also risks pulling tiles out of line while the bed is still soft. The wait is genuinely the boring part of the job and there is no way round it.
Do I grout the corners too?
No, and this one catches nearly everybody. Internal corners, where two walls meet and where the wall meets a bath or worktop, get flexible silicone rather than grout. Those junctions move independently as the building and the fittings expand and settle, and grout is rigid, so it cracks along the corner within a year and then lets water past. Grout the flat field of the wall and silicone every internal angle.
What size notched trowel do I need?
It scales with the tile, and the adhesive tub states it. Small tiles want a fine notch, large format tiles want a deep one because they need more adhesive to bed into and are less forgiving of an uneven wall. Using too fine a notch on a big tile gives partial contact and hollow spots, which sound obviously different if you tap across a finished wall. Larger tiles on anything less than a flat wall generally want the back buttering as well as the wall combed.