What did Severus do before he became potions master at Hogwarts.

We just have to use our imaginations there because Rowling has not specified what he did. I think his working in an apothecary is a valid suggestion, though it's not a job he would have held long because remember, he was looking for a job at the Hog's Head when he overheard the Prophecy.

But yeah, chemist in an apothecary is my first vote.

He must have spent his time training as a Potions Master. I can imagine easily spending 3 o 4 years working and learning as an Apprentice with another established Potions Master.

At the same time, he would have continued sharing "activities" with the charming brotherhood of Death-Eaters. Well, I imagine.

Perhaps he was being sponsored da a high ranking death eater and getting further educated in the art of potions making.