Help with fitting?

[Allen Smith]

Unfortunately, there's no automatic detection of how one piece fits on another. So for a non-painful buiding experience, you need to build on the grid. If the first piece you placed wasn't aligned, nothing else will be. To check that, select your initial piece, then go to Tools > Show Inspector. If the Location xyz fields aren't whole numbers, you have found your problem. You can fix that by moving all the parts in the model back to the origin by doing Edit > Select All, then Edit > Move, and entering the necessary offset to correct your position.