<br>Stunnel has the same requirements for certificates, for both the client and the server configuration. So, technically you generate the certificates in the same way for both cases.
<br>I'm not sure if you have a specific requirement for a certificate with .cer format/extension.
<br>I have found the following document very helpful to understand the basic workings of stunnel
<br>There is also a batch script that will help you with the creation of certificates and some reading of the code, paired with openssl documentation will certainly put you in the right direction.
<br>Thank you very much for your attention and have a nice day