Once you are done generating it place <your-family-name>.key and <your-family-name>.crt in the <ip2ddata>/family folder (for example ~/.i2pd/family).
If you want to register your family and let the I2P network recognize it, create a pull request for your .crt file into contrib/certificate/family.
Certificates added into the public repository this way will appear in i2pd and I2P next releases packages. Don't place .key file, it must be shared between you family members only.
Run i2pd with the parameters 'family=<your-family-name>', and make sure you have <your-family-name>.key and <your-family-name>.crt in your 'family' folder.