If the link to official WDS guide is too much complicated for you, try to have a look at this:
http://sujianto.files.wordpress.com/200 ... krotik.pdf
It is quite old but shows the screens with explanations.
Also check this one, it looks pretty good and is more recent:
http://harry.subnetworx.de/2014/02/21/r ... ion-guide/
Did you even try to google:
https://www.google.cz/search?num=100&ne ... LMFz962Z0E?
Now you have to try yourself.
Anyway, remember that the best "wireless" link is only with wire.
So, do as much as possible to connect additional AP by wire instead of using WDS. Or at least use PtP bridge. It costs 3times more then adding one device to WDS, but will provide speed and reliability of the connection. WDS is always a trade-off.