Find a job (part time or full time) that pays in Bitcoin
Start a business and have Bitcoin as the major payment system.
Do freelancing works that pays in Bitcoin
Offer whatever skills you have and accept Bitcoin as payment
You can mine for Bitcoin
Last but not the least,
You can also buy Bitcoin in exchange of fiat.
Consider Bitcoin as the fiat money you have. How do you Increase (earn) fiat?
It's the same for Bitcoin.
Absolutely great advices to increase bitcoins on hand ,but I think we can add even if the jobs are not paying bitcoin but the profit or payment he may have will be bought or convert to Bitcoin as well with this everything he will do is for the increase of his owned bitcoin
Learn technical analysis and to read the chart and start trading.
If you think bitcoin will go down, either short it or exchange your btc to any stable coin like tether.
Buy again if you think it has hit the bottom.
If you are referring for Daytrading then I will definitely disagree because that area is not for everyone and as I have read in another thread the smallest percent of members who tried Daytrading that succeed besides the larger fails