Lester Crest offers Franklin the opportunity to perform assassinations for a money, these are straight forward Jobs where you eliminate a specified target and then get paid. Each target's death will drastically affect the stock markets which means if you can buy and sell at the correct time you stand to make a lot of money.
Hotel Assassination
Before the mission purchase Beta Pharmaceuticals (BAWSAQ). Then sell Beta after the mission for 80% profit. Then wait at least three days in-game and purchase Bilkington (LCN), then wait a week or more for it to rebound and sell it before the second mission for 80% profit.
The Multi Target Assassination
Before the mission purchase Debonaire Cigarettes (LCN). Then sell Debonaire after this second mission for 80% profit. Then purchase Redwood (LCN). Do not do the third mission yet, but instead advance the game for two days then sell Redwood for 300% profit.
The Vice Assassination
Before the mission purchase Fruit (BAWSAQ). Then after the mission sell Fruit for 50% profit.
The Bus Assassination
Before the mission purchase Vapid (BAWSAQ). Then when the mission has been completed invest in Vapid. Do not do the fifth mission yet, but instead advance the game for two days then sell Vapid for 100% profit.
The Construction Assassination
Before the mission purchase GoldCoast (LCN). Then after the mission sell GoldCoast for 80% profit.