If your 2012 accounts are not yet audited/published then it is simply passing a journal entry with effect from 31 Dec 2012 (assuming your financial year end is 31 Dec) as follows:
Tax expenses Dr (P&L Account)
Provision for tax Cr (B/S Account)
Being the provision of tax for 2012
However if you have already published the accounts then it has to be accounted for as a prior period error. The general rule for prior period errors as per IAS 8 is that you must correct all material prior period errors retrospectively in the first set of financial statements authorised for issue after their discovery (vide. IAS 8.42). That means in 2013. Journal entry would be:
Retained Earnings/Accumulated Profit Dr (P&L Account)
Provision for tax Cr (B/S Account)
Being the correction of tax under provision in 2012
You also need to restate the 2012 comparative figures.
