[vmception]

Everyone that is saying just "put the earnings on Schedule C or Schedule 1 of your tax form" is overlooking the part about your side project getting you sued and you having much more to worry about than just taxes.

The LLC (and other entity types) increases the deterrent of this ever affecting you.

The LLC specifically has some theoretical weaknesses, but the reason it is a deterrent is because it deters people from spending more money to find out about those weakenesses, and it also increases the odds that people won't be able to know where you are or serve you properly. The more expensive it is to find out anything, the less potential aggrieved people will bother, and "finding out" for them could mean "even harder to create liability and a payout", so they get bounced, or deterred, from trying.