Maybe I'm missing something but how did Don get to where he is now? Obviously he is v.smart and charismatic, but one has to have the right opportunities to get to Sterling Cooper.

So what's Don's time line? Army, then college while working as a car salesman, then ...? (I vaguely recall Betty mentioning that he was some sort of saleman/ corporate guy when they met while she was on the fur coat modeling shoot.)

Could someone fill me in with bits and pieces revealed over the episodes, plus your own theories to fill in the gaps? Thanks in advance.

I don't recall that "Don" went to college. The real Draper did, so maybe he has a copy of the diploma, or got the original from Anne, along with other Draper ID.
As to how he got into SC, your guess is as good as mine. I don't think they've ever explored it, but I've been curious as well.

They explored it this season.  3 or maybe 5 episodes back.  Don met Roger somehow, and they ended up at dinner together.  Don got Roger smashed and kept bugging him for a job at SC.  Roger "sort of" agreed to let Don have a foot in the door.  Don just showed up for work the next day. 

I'm sure I mangled the story a bit but that's the rough idea.  It was very interesting to see Don sucking up to Roger back then.  It was probably a flashback in an episode when they were at each other's throats.

Episode 6, In a series of flashbacks, as Roger is working on his book.

Don is working as a sales person at "Hellers Luxury Furs" (He worked there when he met Betty).  Roger came in to buy a "getting to know you" mink cape for Joan. 

Don asked Roger for a job at S.C., Roger said no, Don was persistant.  Later he talked Roger into having a drink with him.  Roger of course got drunk.  That was all the opportunity Don needed.

Next day Don showed up at Sterling Cooper ready for work and managed to convince Roger that he had hired him.

Final scene, shows the two on  the up elevator, Roger with a bewildered look on his face, Don with grin.  The Doors close,  and we hear Skeeter Davis singing "The Ladder of Success"

Don wrote in his journal that was sorry he didn't  finish high school.
In one of the interviews he said  something about night classes at a city college.