Best way to do this is wait for Lester to invite Franklin to undertake assassination missions. It is pretty simple the missions are designed to protect Lester's 'interest in the stock market.
During the first mission you go to the pier sit down with Lester on a bench and he gives you the target and advises you which company he is going to buy shares in.
All you have to do is go onto your phone and use the internet, hit money services at the top, then find the relevant company and sink all you money into the same company as Lester.
Complete the mission and then wait for one day to pass go back into the share dealing on your phone, as a result of your hit the value of your shares should have risen significantly sell all shares you hold and trophy unlocked.
Thanks