I would start with the seatpost & saddle, headset, fork, stem, bottom bracket, crankset.

Then derailleurs and brakes (installed but not adjusted)
Then wheels *with tires, tubes, and cassette already installed), handlebar, shifters/brake levers
Then install the chain and set the alignment (front) and end points (front and rear) of the derailleurs.
Then install cables
Then set up the derailleurs and brakes
Then get the shifters and brake levers set where you want them.

Then install grips or tape on the bars.