Make sure the bench is flat, and check that the boxes etc are all square. A QE is good for checking the boxes are square. Take your time and enjoy.
Yes !!!
And make very very sure you get the beespace right - that is as important as it being flat and square.
You should aim for an inaccuracy of less than a millimetre!
It does NEED to be done accurately. Not fast.
Before I switched to Paynes poly (total assembly is seconds per box to fit the rails - if they've remembered to pack them! - and unless you've decided to go TBS like Erica … ), I had discovered that it was easier to assemble National boxes* using the Maisemore instructions than the Thorne ones. Just doing things in a different sequence.
Both sets of instructions can be found on the companies' websites - useful to see what you are letting yourself in for!
* Whether DN, shallow or 14x12, all national boxes go together the same way.
Only difference is the height of the walls, and how many fasteners you can be bothered to hammer/screw into those sides.
I used frame topbars in place, and 'drifting' their supporting sidewalls up and down to get the beespace right. But I know I'm rubbish at carpentry, so I'm slow and careful.
Shouldn't be any rush whatsoever to get building. Not least because they take up much more space after being assembled!
Most folks would be waiting for the New Year Sale, and a few bargain 'primes' plus the near-certainty of lots of flatpacks in 'second' grade cedar (which is generally plenty good enough). And frames in bargain 50-packs (not too many really, even for a beginner) again in highly usable second timber.
Last point. Don't forget that you need some spare kit for swarm control. Sales are useful, but (perfectly understandably) don't happen at the time that people discover that they REALLY NEED more kit. So forethought and planning ahead can save you lots of money.