The CIL process begins when a planning application is submitted, you need to submit 'Form 1: additional information' with your planning application.
The additional information form asks for details of new floorspace to be created by the development, and details of any floorspace to be demolished, which can be offset against the new floorspace calculation. It is in your interest to provide this information, especially where there are existing buildings in use on your site, in order to ensure that the CIL calculations can be completed correctly.
If the application is CIL liable, 'Form 2: assumption of liability' will be required at the same time.
In some cases, proposals liable for CIL can be commenced with Permitted Development. When this is the case it will be your responsibility to inform the us using 'Form 5: Notice of Chargeable Development' before commencement begins.