I put together a voodoo child’s bike from Halfords before Christmas. It came in a flat box, very well wrapped, with plastic discs and things attached to save damage from handling. Front wheel was off, rear wheel on. The crank arm was through the spokes in the wheel and fastened with cableties, it needs disentangling carefully to avoid damage.
It had cable discs, one was scraping slightly so I had to reset the position of the adapter on the frame. Gears were slightly off, but turning the barrel adjuster was sufficient, nothing more.
No more than I expected, around 1.5 hours for an amateur taking their time, an expert would be quicker. A degree of knowledge helped. Photos / google images of what the bike should look like might help as well if you aren’t sure.