This is pretty simple.
First, you need 2 virus scans.
These you can get at: Jotti's malware scan and VirusTotal
Next, you need screen shots or a video. If the plugin are some commands, you may get a screenshot. HOWEVER, if the plugin gives you *for example, rocket boots, or Iconomy, or any plugins that can't be proved through screenshots, you must get or make a video.
Then, You must describe what the plugin does, for example, the rocketboots give you the ability to fly. Or if it's a command plugin, you must list the commands like this:
Afterwards, if this is your creation, leave the credits to sayCode:Commands /god /heal /etc
If not, then leave the person's IGN. Like this:
DO NOT leave the website origin. That will be considered advertising, and it will result in a ban.
Finally, add the attachment.
Most people have a hard time finding the button. Just scroll down and click this:
Follow these guidelines and you should have no trouble.