The HOWTO explains this quite nicely - did you read it?
on the server side it boils down to commenting out the user auth part (auth-user-pass-verify or some plugin)
on the client side you'd replace 'auth-user ....' by
cert client.crt
key client.key
that's it. Instructions on how to generate client.{crt,key} are in the HOWTO as well, using the 'easy-rsa' script. The script you want is 'build-key'
