We already have the same topic[1] but mainly for bitcoin investing. If others are interested in reading that thread, they're free to visit it.
[1]
Reasons to use only Discretionary Income for Bitcoin investment.
Cutdown on your basic expenses if you can, get a better paying job, get a side hustle, learn profitable skills, do what you can to earn more than your basic expenses.
If someone who's used to their lifestyle and can't cutdown the expenses. You've already mentioned the main requirement of how to do it, and that's to add more income and do more side hustles. Increase the income or profit that you'll get from various sources.