I never worked as freelancer, but I'm thinking to start as far as I keep studying, and I was wondering... How do freelancers charge their client when they get paid x per hour?
How do they assure the client they worked for a certain amount of hours?
Do they do it by prediction or they say they'll work y hours per day?

Feels like a dumb question...

Obligatory tag: @JoshBent

A freelancer that values his health and his experience will rarely work by pure hours, except if you're with fixed clients - because if you are, then the software usually records your screen or they will try to dump down the price and more, basically you want to set an amount for which you'll feel like doing it, based also around how much money they will get, with optional further budgets for changes and per sale profit.