YouTube videos! There is so much to Build/Buy mode, I could tell you tons of things you already know and maybe a few you didn't, and probably a lot that you wouldn't understand unless you saw it demonstrated. But, some basic things that everyone needs to know, but maybe don't:
You can change the size of things with the square bracket keys, []. Or, if you aren't from the United States, whatever the next two keys are to the right of "P".
Hold alt while moving something, and you can place it anywhere, instead of it snapping to the grid.
The cheat "bb.moveobjects on" will let you put things wherever, even if the game doesn't like it. Be sure to play test after using that, to make sure things still work! Sometimes things will be inaccessible after using that.
bb.showhiddenobjects shows various things that usually don't show up in BB.
bb.showliveeditobjects shows even more things. You need to use showhiddenobjects first.
bb.ignoregameplayunlocksentitlement will let you have access to things you normally have to earn, like career rewards.