Hi, Mad, Ditto everything Draboo said, especially the bar clamps or 'C' clamps, and the waxed screws. (And soap is good as well, like Gomer said.)
I don't know how much of the following is obvious or not, but maybe there's some help in it, so...
Make sure to establish a 'baseline' on the wall where the bottom of the cabinet will line up with. Level it. If the ceiling is kinda crooked (they sometimes are) you'd rather have a level bottom line. (In other words, don't measure off the ceiling.)
Use a studfinder to locate the studs before you do anything. Then attach 2"x2" pieces under the shelves, (since these won't be attaching to an overhead soffitt) measure out where the screws will hit the studs, and prestart 'em like Draboo said. Reason is, once you've got that heavy sucker in place, you wanna just go wham-wham-wham with the screwdriver.
I'm sure you know not to screw anything into drywall and expect it to stay (okay, I know, obvious!!

)
On screws, the coarse-threaded drywall screws will really 'grab' when they go into a stud. 3-1/2"-ers will do nicely.
Can't think of anything else off hand. Hope this isn't TOO redundant!