This is fairly easy to do as long as you do this at the right time in the game.
During the story you’ll unlock Lester Assassination missions for Franklin.
Before you start this mission you should invest in Beta Pharmacuticals (BET)
The very first mission will have you kill the CEO of a rivalling company.
You should then invest in Bilkington Pharmacuticals (BIL)
After the mission is over you should quick save.
Your shares of Beta Pharmaceuticals should skyrocket soon after.
To get you shares of Bilkington Pharmacuticals (BIL) to increase you can speed up time by sleeping in a bed (Trevor’s sleep is 12 hours & is most effective for this). Keep sleeping & saving until the stock hits a peak and then sell for maximum profit.
Hold off from doing the rest of the assassinations until after the main story for maximum profits.