This article will go into detail on how to generate certificate signing request on Google App Engine.
To create the CSR , execute the following command in openssl.
openssl req -nodes -newkey rsa:2048 -keyout myserver.key -out server.csr
This creates two files. The file myserver.key contains a private key; do not disclose this file to anyone. Carefully protect the private key. Once the CSR has been created then you should submit it to the CA to get the CA bundle and the domain certificate.