Lay everything out first
Before a single screw goes in, every panel and bag of hardware should be laid out and checked against the parts list. Flat pack furniture ships with more small pieces than most people expect, and a missing bracket found halfway through the build is a real delay.
Manufacturing flaws happen too, a warped panel or a stripped hole, and it is easier to flag those before assembly starts than after a piece is half built.
Follow the order, not the instinct
Flat pack instructions put steps in a specific order for a reason, usually because a later step needs a panel to be loose enough to adjust. Tightening every screw fully as you go instead of leaving things slightly loose until the end is one of the most common mistakes that leads to a frame that will not square up.
Bunk beds, wardrobes, and anything with multiple connected panels are the pieces most likely to punish a rushed order.
Two people for the bigger pieces
Wardrobes, bunk beds, and large bookshelves genuinely need two people, not for speed but because some steps require holding a panel steady while another goes in at an angle. Trying to do it alone often means propping something with a knee or a random object, which rarely ends well.
Crib assembly always gets a second set of hands and a final check of every joint, since a loose bolt matters more there than almost anywhere else in the house.
Checking the finished piece
Once built, drawers should slide evenly, doors should close without catching, and the whole piece should sit flat on the floor without rocking. A slight wobble usually means one leg or panel was not seated fully rather than a fault in the furniture itself.
Give a new piece a light shake test before loading it up with books or clothes. It is easier to fix a loose joint before the weight goes on than after.
When it makes sense to hire it out
A single bookshelf is a reasonable weekend project. A bedroom set with a bed frame, two nightstands, and a dresser arriving the same day as a move is a different scale of job, and that is usually when it makes sense to have it built by someone who does it daily rather than losing an evening to an instruction booklet.