Functional test procedure for RoR Web portal?

I am going prepare functional tests from user perspective but my software is not ready yet, a Ruby on Rails web portal give sensor device real time information with lot of reports and sensor devices can be controlled from web portal. How can I write a functional test preparation for something that doesn't exist yet?

Let say, If developer saying Web portal has feature like security, load balancing, scaled, bla bla... Can any one please describe, what is most effective way for functional testing as standard point of view?