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 “Small”): 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 charged by a flat fee.
Full EASA Validation Support - with need for technical support (“Category “Medium”/”Category Large”): Your EASA approval will be forwarded to the responsible technical section. An EASA PCM will be allocated to 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 by a flat fee. Please consult the EASA Fees and Charges Regulation Commission Implementing Regulation (EU) 2019/2153 for the applicable flat fee.