Each BB is different.

For a Euro, both of the cups thread backwards from the top, into the frame; thread one cup in (with either a very large crescent wrench or a pipe wrench, and don't forget to grease the threads), put the spindle in about halfway on that side, put the spacer onto it, and put on the other cup.

For an American, you simply pound or press the cups in (rubber mallet, or 2x4 and hammer), insert one of the bearings into a cup, put the spindle halfway through, put the spacer on the spindle, and then put the other bearing in.

And for a Spanish or Mid, put a bearing in (you should be able to press it in with your hand, after you grease it), put the spindle halfway in, slide the spacer on, and then put the other bearing in.