Hello
I am trying to use cert_issuer on a server, but I don’t know how to use the USB on which I store my private key to sign certificates.
Hello @ely 
First; make sure the USB device is attached available on your server.
then next; set cert_issuer to look for the private key file on the USB drive. You may need to provide the precise path to the secret key file in your cert_issuer setup.
make that the USB drive permissions allow the server to acquire data from it.
Once set up cert_issuer should be able to verify documents using the private key stored on the USB drive.
I hope this will helpfull for you 