Have you ever had to write documentation for a product you've never used? I love it when I get that type of assignment. That might sound crazy, but it's a special opportunity to write documentation for a new user. As you master a product, you tend to forget what the starting difficulties for new users are.
As a new user to a product, you will get a fresh perspective. So, learn the product and take notes as you do it. That's the first step.
Once you have a basic understanding of the product, write an outline or table of contents. Put your notes in the relevant sections and use those notes to create vignettes to appeal to the product's users and help them learn the product.
Then, you can learn the product for real and finish working.