In the past you would receive a payslip/reminder in the post from HMRC, that could be used to pay your tax over the counter in a bank.
Unfortunately these are not going to be issued for the tax due in January 2016, if the return has not been submitted by the end of December.
This means that the tax due can only be paid by the below methods:
1) Using online banking, making a transfer to HMRC’s bank account.
2) Using a debit card online at this page: https://www.tax.service.gov.uk/pay-online/self-assessment (they do also accept credit cards, but these are subject to a 1.5% fee)*
3) By cheque, but only by posting this to HMRC. A payslip to accompany the cheque can be printed off from this page: http://www.hmrc.gov.uk/gds/payinghmrc/payslip-sa1.htm. You will also have to allow time for the payment to arrive before the 31st January deadline, and should obtain proof of posting in case of any problems.
* One point to beware of is if you are subsequently due a refund of tax, this will then be credited to the card that you used to make payment – which could cause issues for withdrawal if this is a credit rather than a debit card.