os x server 5.1.7 website redirection question

I can not figure out how to make an external redirection from Domain www.a.com > www.b.com with os x server 5.1.7 running on el capitan 10.11.6.

Is there a step by step guide?

The web server is up and running and the www.a.com is served by the server, but i need redirection...

any help would be appriciated

Joe