To make your company dormant, you must contact your local Corporation Tax office and tell them that your company is not trading and has no significant accounting transactions - i.e. your company is not receiving any income from any source and it is not spending money through the business bank account.
The contact details for your local tax office will be printed on the letters you have received at your registered office from HMRC. You will hear back from them in about 2 weeks with their decision and any actions you must take.
if your company carried on any business activities prior to becoming dormant, you will have to complete a Company Tax Return for that period, even if you don’t have any Corporation Tax to pay. If your company has never traded, you may not have to do anything until you begin trading.