The procedure you must follow depends on the status for beneficiaries of the Withdrawal Agreement you wish to obtain:
You can obtain a residence status if you are a UK national or a family member who already resided in Belgium before the end of the transition period in accordance with Union law or if you join a beneficiary of the Withdrawal Agreement as a family member. This status allows you to continue your stay after the end of the transition period.
You can obtain a local border traffic status for beneficiaries of the Withdrawal Agreement if you were a British frontier worker in Belgium before the end of the transition period.
The procedure is different depending on whether you had already registered before the end of the transition period and you already hold a valid residence document (annex 8, 8bis/E/E+/F/F+ card) or a valid local border traffic document (annex 15), or not yet.
Below you will find the procedure applicable depending on your situation.