1st thing is to go to the budget section by simply clicking the amount of money you have

there should be a pop-out menu that shows you the budget of your property

there's also an option to turn on the summary at the end of the day, tick that option

now wait till near midnight, I repeat, NEAR MIDNIGHT e.g 23:55

Now open your budget menu again and there should be an button which will let you adjust the rent (2nd button from the right)

Now change the rate of each property Rent to 200% then let your game run

At midnight (00:00) the Budget menu will automatically pop-out and when this happens, reduce the rent rate to normal (100%)

Repeat this to gain an extra boost for your income, especially at the beginning