Decide the layout first, then the products
Layout drives plumbing, electrics and tiling quantities. Choosing a vanity before the layout is settled means re-pricing everything when the layout changes.
Set a decision deadline for layout, then a second one for sanitaryware, then tiles. Late tile changes are the single most common cause of a stalled bathroom.
Understand what has a lead time
Standard sanitaryware and popular tile ranges are usually stocked. Bespoke vanities, specific stone, imported ranges and unusual sizes are not, and lead times move.
All materials should be on site and checked for damage before strip-out begins. Starting a job while waiting for a delivery is how a one-week job becomes three.
Living through it with one bathroom
Expect the water to be off for parts of some days, and expect the room to be unusable for the full duration. Arrange alternatives in advance - a relative, a gym, or a temporary WC where practical.
Agree with your fitter which days will have the supply isolated so you can plan around them.
Managing the last ten percent
Snagging is normal and worth doing properly. Walk the room in daylight, check silicone lines, door and drawer alignment, sealant at the tray, tap operation and extractor performance, and write one consolidated list.
A single organised list gets cleared in one visit. A trickle of texts over three weeks does not.