In the product detail page, click the “CUSTOMER REVIEW” section, then click the “WRITE A REVIEW” button, Scroll down to see the entire page -- the review form would show up.
After finish the form, click “SUBMIT REVIEW” button. --as below image: 
* Note: you just can write the review for the item/ items that you have purchased.

 

Step-1 :

Step-2 :