Upon clicking either of the 'Post Ad' links you will see a page like this:

Subject: Fill in any suitible title for your ad here.
Message Icon: Play with this or ignore it as it pleases you, it's not important..
Add BBC Tags: (See above)
Message: This is where you write all the info you need to include in your ad! I don't think I need tell you what to write here, but do be aware that if you are registered on the forum in the interests of your personal safety you need not include any personal contact details.
If for any reason you don't wish to register on the forum, you will have to leave some form of contact details. You can read more about registration here.

Aye: That's it, you're done, just click 'Post'. Nice.