One of the biggest challenges is making sure wall cabinets are installed flush with each other. One trick that helps a great deal, particularly if you are doing this by yourself, is to install a temporary rail across the wall under the wall cabinets. This gives you a place to rest the cabinets as you screw them into the wall and to each other while also helping assure they are at the same height. The second trick I'd suggest to help make sure adjacent cabinets are aligned with each other is to buy a couple of Vise-Grip locking C-clamps with swivel pads. If your cabinets have face frames make sure you buy clamps which are deep enough to go around them. Prop a cabinet up on the rail next to its neighbor, get the clamp adjusted so that it's almost tight, finalize alignment, snap the clamp shut to hold position, and screw the cabinet in place. The human eye is amazing at picking up tiny misalignment errors and this is a great way to quickly make them go away.