First, you need to define under which regulation you want and can certify your medical device: MDD 93/42/EEC or MDR 2017/745. Then you need to define which class your face masks are. Rules to define it, you can find on the following links:
Classification rules under the MDD you can find in Annex IX
Then you need to prepare the Technical documentation. Which documents are necessary to be in Technical file under the MDD you can find in Annex 1. Required technical documentation under the MDR you can fin in the
After you prepare a technical file, you need either to register your device in local authority (if it is class I and do not require Notify body) or you need to certify your medical device with Notify body.
For more information about MDR, please see the following articles: