i fitted a B&Q kitchen when we moved, had the original 1930 kitchen in, so nowhere for cooker, and whilst very nice not practical, so stripped back, someone plastered and was left with 2 square rooms with 2 pipes and a waste outlet.
the doors still are not all straight (will get round to it, only needs a fettle on the hinges...) and the worktop are butt jointed rather than routed together.
so some things from someone who had NEVER looked at anything like that before.
1 - your kitchen will not be square or level. Granted mine is a 1930 semi so not expected, but don't assume anything or use anything other than the same spirit level to get it straight.
2 - Make sure you have a reliable and good quality spirit level that has a good size to it, you need it to rest and not move.
3 - get some easy to use clamps, so when you screw the carcases together they will not move.
4 - make sure your cutting tools are suitable, so jigsaw for the sink doesnt have a bit of damage that scratches the worktop (grrr)
5 - I got a Lidl circular saw for the worktops, worked well, but easier to clamp a straight edge and use that for perfect straight cuts
6 - Beware of white good sizes, some washing machines are slightly more than the (i thnk) 65cm worktop size, so beware that may stick out.
7 - measure and measure, and measure, and mark then walk away, then re-measure.
8- base ALL wall markings from a middle mark and use straight edge level for the carcass estimate. Our kitchen floor was around 3/4" higher at one end than the other (at a 3.5m length). we measured a mark at one end and the other and was stumped for a while as it wasnt straight then worked out the floor was the issue. We then marked the wall from the middle, marked the wall with straight edge to ensure it was what we wanted, then could level to that mark one carcass,when happy, did the rest.
9 - get a trade card from B&Q, saves a fortune if you are buying from there, just need a business card or flyer when we did
10 - Factor in extra costs for odds and sods, so you will mis-drill a door handle hole, or run out of screws.
11 - it will be finished one day!