Check out the PRO center on Houzz to help you with how to use Houzz:
http://www.houzz.com/proCenter
There's a button on your profile that you can press that says "get reviews" that will help your clients through the process or a paper form which is available on the link above.

Agreed with Bill Fry, the PRO center is an awesome resource for realtors and other pros!

I know it has been said but I would check out the PRO section and if I was you I would start a PRO account. Once that is done there is a profile menu on the left hand of your screen below your profile picture and follow button. Click on the review button and it will take you to the review page the get reviews link is located right below your name. From there you add your clients emails etc.
But this video explains things well: https://support.houzz.com/hc/en-us/articles/202776578-How-do-I-request-reviews- (Plus in this video it stars Bill Fry Construction)
You can also have clients fill out paper reviews, which we are going to be doing with some of our customers that do not want to sign up for another social media site. Also below your name on your public profile there is also a review me option.