The step by step guide below gives out detail on how to get up to 1.5 billion from these missions by playing the stock market.
Few Important Things to Note:
DO NOT DO ANY OF LESTER'S ASSASSINATION MISSIONS AFTER THE FIRST ONE!
These steps are very time sensitive so don't waste too much time between missions or you might miss an investment window.
Be sure make use of multiple save files and make sure to save before each investment or each assassination mission because the timing on some of this can be a bit tricky and confusing.
For a condensed TL;DR version scroll down to the very bottom.
Lester Assassination Mission #1 - Hotel Assasination
This will come naturally being that you cannot progress through the rest of the story without having to do the first one. Invest in [BET] BettaPharmaceuticals (BAWSAQ) with each character before starting the mission with Franklin. You won't have much money to invest so the take won't be that big. Im not exactly sure what the max return percentage is on the investment because at the time I did the mission I wasnt that familiar with how the stock market works. If i remember correctly though it caps out at around 80% profit at least.
DO NOT DO ANYMORE OF LESTER'S ASSASSINATION MISSIONS:
There are probably other ways of manipulating the stock market and making vast amounts of cash without these missions but if you do them early on you will miss out on being able to invest your take from the final heist on his stock tips and thus not be able to make vast amounts of money as quickly.
Finish all the main story missions besides Lester's assassination missions.
Once you finish the last story mission you get the money you made from the last heist you did at which point you can get on the road towards being a billionaire.
Lester Assassination Mission #2 - The Multi Target Assassination
BEFORE accepting the mission invest all the money you have with each character in the stock [DEB] Debonaire (LCN). Immediately after this mission is completed the stock price will start to rise. It will take around 8 in game hours to reach its peak at 80% return percentage at which point you can sell all your stock with each character. If you wait too long to sell then the return will be significantly lower so keep an eye on the stock and wait for it to reach its peak and make sure to sell before the next trading day which begins every weekday at 8am in game time.
Here are my numbers to show you what I made with Michael: $32, 081,246 invested. $58,714,376 return.
Invest in Redwood
This is the most important step and the step most people are not aware of. Even though you are meant to hurt this company with the last assassination mission they come out on top in the end. Invest all your money with each character in [RWC] Redwood (LCN). With any of your characters advance the in game clock by 48 hours past the last trading day which started at 8am by going the the save screen at any of safe houses and then backing out immediately. Each time you back out from a save the clock advances 6 hours. If you wait 2 days and land on a weekend you have to wait additional days to get to the next week as stock prices do not move on weekends. Once you get to start of the second trading day Redwood's stock will be up by a whopping 300% instantly at which point you sell.
Here is what I made with Michael: $58,714,376 invested. $234,839,456 return.
Lester Assassination Mission #3 - The Vice Assassination
BEFORE starting the mission invest all your money with each character in [FRT] Fruit (BAWSAQ) and then complete the mission. You will net a max of 50% return from this stock at which point sell it on all your characters.
Here is what I made with Michael: $234,839,456 invested. $353,735,360 return.
Lester Assassination Mission #4 - The Bus Assassination
Complete the mission as you would do normally. AFTER completion invest all your money with each character in [VAP] Vapid (BAWSAQ). Advance the in game clock 2 days and at the start of the second trading day the stock will be up 100% instantly and will not peak any further at which point you sell.
Here is what I made with Michael: $353,735,360 invested. $707,470,720 return
Lester Assassination Mission # 7 -The Construction Assassination
BEFORE starting the mission invest all your money with each character in [GCD] GoldCoast (LCN) and then complete the mission. You will net a max of 80% return from this stock at which point sell it on all your characters.
Here is what I made with Michael: $707,470,720 invested. $1,282,880,384 return.
Ramez05 also shared a Condensed Version of his above guide:
BEFORE the second assassination mission invest in [DEB] Debonaire (LCN). Sell at 80% return.
Invest in [RWC] Redwood (LCN). Advance in game clock 2 days. Sell at 300% return.
BEFORE the third assassination mission invest in [FRT] Fruit (BAWSAQ). Sell at 50% return.
AFTER the fourth assassination mission invest in [VAP] Vapid (BAWSAQ). Advance in game clock 2 days. Sell at 100% return.
BEFORE the fifth assassination mission invest in [GCD] GoldCoast (LCN). Sell at 80% return.