Ok, first place to go is to here:
http://cityark.medway.gov.uk/query/results/?Mode=Search&PathList=/Z4a_Medway_Ancestors/Scroll down to the bottom of the page, and you will see a list of the parishes that the database covers.
In this case, we want Frindsbury, All Saints, so click on the 'click here' hyperlink.
This will show you a page of all the records for Frindsbury All Saints. We want marriages in 1853 which is result number 16. This is a scan of the register of marriages between 1848 - 1855. Click 'view images'
You then have a list of scanned pages - unfortunately, the names of the pages do not give away the date so you have to estimate and then work forward and backwards! Stephen and Sarah's marriage is on scan 00000095.
Click on that, the page should load and their marriage is the second one (bottom left)
Hope that makes sense!