Go to the page for the item you wish to post a review for.
Here is a link just as an example:
On the right hand side there is a blue box that has three tabs "Add Review", "Picture" and "Video".
You can type or paste your review into that little box, then click on the "Add a review for ..." button.
Or you will also see there is a link for "Big Post Screen" which gives you a little bit more space to type.
Once you have posted a review you can still edit the review. Personally, I post a short review then use the 'edit' function to write a full review, add images and other fancy stuff. The edit window is just easier for me but everyone has their own style.
Hope that helps. Cheers :)