Open Internet Services Manager from your Administrative Tools.
Open the Properties for the Web Site that is hosting OWA (normally the Default Web Site).
Select the "Directory Security" tab and click "Server Certificates".
You will now be presented with the "Pending Certificate Request" dialog box (below), select "Process the pending request and install the certificate", click Next.
The "Process a Pending Request" dialogue box will appear (below), navigate to the site certificate that you received. click Next.
You will now be presented with the "Certificate Summary" (below), click Next.
Install the intermediate certificate :
You have now installed the SSL certificate into our web site. Next enable SSL for OWA.
Using the Internet Services Manager, open the properties for the "Exchange" virtual directory.
Select the "Directory Security" tab and the click on the "Edit" button in the Secure Communication section.
In the "Secure Communications" dialog box (below), check the box "Require Secure Channel (SSL)". You can also check the box "Require 128-bit encryption", if you do check the 128-bit checkbox, any browsers that do not support 128-bit encryption will be unable to connect to OWA.
When users enter http://ahost.adomain.com/exchange, they will receive an "HTTP 403.4 - Forbidden: SSL required https://ahost.adomain.com/exchange. Please see the Microsoft article regarding forcing the use of SSL with OWA
Finally, ensure that your Firewall is configured to allow HTTPS (port 443 by default) to pass through.