I will assume you know how to use IIS and request and install SSL.
1. On one of the front end server, request and install SSL.
2. Open the IIS manager, find a "exchange" virtual directory and set Require SSL and Require 128 bit encription.
3. On Exhange Systems Manager, open the front end server.
4. Go to the protocol--> http, right click the http and open the properties for the Exchange Virtual Server.
5. Click settings tab, and check the "Enable Forms Based Authentication"
6. On the front end server, reset the iis or restart the server.
7. Try connect the owa using https://
8. On the front end server, start the IIS manager and export the cert.
9. on the second front end server, import the cert.
10. repeat #2 to #6 on the second front end server.