You should receive a notification informing you that your match fee is due. 

You can either click that notification or, at any time you think you might have an outstanding fee, head to 'payments' then 'your payments'.

Select the game you need to pay your outstanding fee for.

Click the PayPal fee button next to 'You Played - Good Game!' then 'Pay with PayPal'.

Log in to your PayPal

And pay for your fee.