Type on one side of the paper only; leave margins all the way around; double-space the text, put two spaces after the end of a sentence. Typographical errors had to be cleaned up with some kind of white-out, wet or dry, and if you had more than three of those on a page, you were supposed to junk it and re-type it.
Each page needed to be numbered, usually at the top right hand corner, and it was a good idea to put your name or the title or both up there, too, in case some fumble-fingered assistant editor dropped a couple of manuscripts and had to try to reassemble them. Hard to do without pages numbered.
If you wanted something set in italics, you underlined (or underscored) it. This told the typesetter to set it in italics, whereas actually putting a word in italics might slip past him unnoticed. This could be costly, because the setter actually had to chose a different typeface, insert it, and then go back to the original.
Your copy editor would mark the em-dashes and all on the ms -- those were done with a space, two dashes, and another space -- thus -- and not an actual em-dash as done in the printed version.
Even with the word processing programs of today, most of those rules remain the same. Most book and movie houses want to see manuscripts in Courier, which is pretty much like the standard typewriter imprint. They want the spacing and margins and double-spacing and all, even though that is a snap to change -- select all and zap it, it's done.
You don't have to double space after a period or question mark or exclamation point because the type is justified and your computer will add a little bit for you there.
Up until now, that's how I did my mss. But given that typesetters don't exist any more, and that whatever I submit is going to be electronic, I'm giving up the underlining for italics. Two reasons: First, whatever wp program they use will collect the formatting enough to show these. Second, if I decide to publish something myself, this saves me the step of having to go through and change all the underlines to italics manually -- because my writing program won't do this on a global search. (It will take out the space-double-dash-space and replace it with an em-dash, so I don't bother with those.)
Not that big a deal, but after you've done it one way for a long time, you have to adjust a little bit mentally.
Damned little mammals are stinking the place up, sucking eggs, and if I had the energy, I'd go stomp 'em flat, but there are so many of them ...