1. You should have OpenSSL installed on your computer. Linux distrubutions should have it installed by default, and a Windows version can be downloaded from here.

2. You should generate a private key using this command:

openssl genrsa -out my-prvkey.pem 1024

This will create a 1024-bit RSA private key in the file my-prvkey.pem.

3. You should generate a public certificate using this command:

openssl req -new -key my-prvkey.pem -x509 -days 365 -out my-pubcert.pem

This will create a public certificate in the file my-pubcert.pem.

4. Then you should upload your public certificate to the PayPal website: