This depends on your work status...
Employees: It's automatically taken from the payroll like tax. HM Revenue & Customs (HMRC) then passes the cash to the Student Loans Company (SLC) every March.
If you earn over £27,295 and repayments aren't being deducted, it's YOUR responsibility to inform your employer. Ensure you keep evidence of having done this as otherwise it can mean you pay a fine.
While we're on this, if you're new to paying tax, make sure you check your tax code – you could be owed £1,000s if it's wrong.
Self-employed / those with other income doing self-assessment tax forms: You're responsible for notifying HMRC of payments due for student loans when you do your self-assessment form (just like you're responsible for telling it of tax due).
If you have additional income of £2,000+ from savings interest, pensions or shares and dividends, this will also be treated as part of your income so you'll need to repay 9% of that too (provided this plus income is over £27,295) via self-assessment.
To pay more: If overpaying is right for you, the SLC has a range of voluntary repayment options.