First and foremost (as always, too!) make sure you are logged into the M.E. user account you would like to create a signature for!

Second, you'll need to find the Your Details link. The Your Details link can be found to the left within the Main Menu. It's below Contact and above Dead Forum.  

Once you have clicked on Your Details, you'll be sent to a page which controls your profile, address, forum signature, connections and various other delicious M.E. things. It'll look like this:

You'll note that right above your profile pick, a little to the right are three grey boxes with links inside them. Community, Edit and connections. You want Edit. :) Click eeeet! A drop down menu will appear with Update Your Profile. Click it! You want that, yes!:

Once you have clicked Update Your Profile, you should be presented with a screen which looks like this:
 

As you can see, right under the forum tab is the place for editing or creating a signature. Just click in the text box, type away and save.

And that's the end of it! :)