You need to generate new keys (or use existing other client/username keys) for the new client/username

On the server:

cd easy-rsa
. vars
./build-key username

On the client:

1. Copy Keys
   

   Copy username.key, username.crt and ca.crt from server to /etc/openvpn/keys/.

1. Edit
   Edit /etc/openvpn/client.conf appropriately to set your configuration (just like server configuration, port, compression,..) and key paths.

   cp -ai /usr/share/doc/openvpn-*/sample-config-files/roadwarrior-client.conf /etc/openvpn/client.conf

2. add to service

   chkconfig --level 2345 openvpn on

3. Start

   service openvpn start

4. Check
   check /var/log/messages if things didn't work as expected

On the server:

cd easy-rsa
. vars
./build-key username

On the client:

1. Install the OpenVPN GUI or the stand-alone OpenVPN client.
   http://openvpn.net/index.php/open-source/downloads.html
2. Copy keys to client
   Copy username.crt, username.key, and ca.crt to C:\Program Files\OpenVPN\config\ on the client.
3. AAA
   Drop roadwarrior-client.conf into C:\Program Files\OpenVPN\config\ as whatever.ovpn and edit appropriately.
   Either use the GUI to start the connection, start the OpenVPN service manually, or set the OpenVPN service to start automatically.

Ideally the client should do some verification on the server key with tls-remote in the whatever.ovpn configuration file.

連結參考OpenVPN Server - Linux