Look at your monthly income, and subtract all of your regular expenses (rent/mortgage, utility bills, subscriptions, food and living costs, etc.) to work out how much you have left to spend.
Give yourself plenty of buffer room – if you think that you have £400 available each month for your car, don’t spend all of it on the car finance payments. You will need to leave plenty for fuel, insurance, road tax, servicing, and so on.
You also need to keep enough money aside to allow for unexpected costs, like a puncture or an unexpected breakdown. It may even be worth having a separate savings account to keep this money away from your regular cashflow in your current account.