First, let me explain what verifying is.  Once you have registered your account with WoWStead you will then have to make a character profile of one of your characters (usually people start with the character that is the main in the guild for who's site they are needing to go to).  Once this is done you need to gain "verify" that character with the guild's site. 

This verifying will show to the guild site admins that you have infact registered on the site.  You will also now be able to post on the guild's forums you have permission for, sign up for posted events, post images in the guild's gallery, etc.

So how do you verify?

First, go to your guild's site.  If your guild doesn't have it's own domain it will be something like www.youguildname.wowstead.com (or like mine it's www.aba.wowstead.com because Azeroth Blunderbuss Assn is just too long).

Once on the site there look in what is referred to as the "net bar" (the bar across the top of the page).  You should see your charactrs name and next to it a red blinkinging "Notice".

Now you can click on the "Notice" and enter your guild's membership password in the pop up.

-or-

You can go to your "Character Management", click on the "Guild" tab, and then click on the "Join Guild's Name".

This will open an area to enter your guild's membrship password.

After that click "Go" and you will be verified.