You need to fill in the EASA form FO.CERT.00041 which can be found here.
There are two different scenarios:
Forwarding Letter only - without need for technical support (category 1): Your EASA approval and related documents will be only forwarded together with an EASA Forwarding Letter to the validating authority. There is no technical support foreseen. This EASA service is free of charge.
Full EASA Validation Support - with need for technical support (category 2): Your EASA approval will be forwarded to the responsible technical section. An EASA PCM will be allocated your task and will prepare the EASA Forwarding Letter. The Applicant Services Department will then submit the EASA Forwarding letter and its related documents to the validating authority. This EASA Service is charged on an hourly rate. Please consult the EASA fees and charges regulation Commission Regulation No 319/2014 for the applicable hourly rate. A financial estimate may be requested at submission of your application and prior commencement of work.
Please also refer to the FAQs on CSV on the EASA website