You will initially have to become registered for Self-Employment using form CWF1. HMRC will then open a self assessment taxpayers record for you and issue a 10 digit Unique Taxpayer Reference (UTR) .
You also need to register as a subcontractor for CIS (Construction Industry Scheme). From April 2007 there will be no registration cards issued. Instead you now have to register with the CIS helpline (call 0845 3667899) who will verify you on their records, and contractors who you work for will use the UTR number to verify you for the 20% deduction rate. If you fail to register for CIS, you will suffer 30% tax deductions from contractors.
There will be no need for you to re-register on a future occasion unless you notify HMRC that you have ceased self-employment on a permanent basis and finalised your SA affairs and now wish to re-start.