Search Influenster for a product you've used - whether a good one or a bad one!
From that page, look for the "Write a Review" button. After that just select your star rating from 1 to 5 stars and start typing your thoughts. Bonus points if you upload a good photo as it really helps other Influensters get a feel for the product.
Also note that you can also quickly write reviews using the Influence menu on the Influenster app!