Every stock has an ID assigned to it going from 1 to 18 in order. They also have a designated "zero point" which is just their ID with an extra zero.
The stock price will always sway around its zero point, with more or less randomness.

So, the lazy method is just:
- Buy any stock when it's below its zero point.
- Sell any stock that is above its zero point.
- Go do something else for a few hours.
- Repeat.

It's a guaranteed but maybe not optimal method that is ideal for lazy longterm gains.

This is nothing revolutionary, and it's even mentioned on the Wiki. However, the only guides I saw on Steam were massive write-ups that lazy people like me aren't interested in.

One note is that the zero point also increases with your Stock Market level (how many sugar lumps you pump into it). It's just a +level -1 thing, which makes it pretty irrelevant. I guess if you're at lump level 10 you're looking for 19, 29, et.c. but that's about it.