Before you can edit a page you need to request an account. Be sure to include your full SCA name in your bio, but don't worry about including much else - you'll get to do that later. Once your email address has been confirmed you will be given an account.
In the upper right corner is the "log in" link. From there you can log into the wiki, or have the wiki send you your password again. If you can not recall you user name, contact Symon of Barnesdale or try finding it on the user list.
Once you have logged in, you can click on your user name at the top of the page. This will either take you to your populace page or to your user page, which will have a link to your populace page. (Newer users will go right to the populace page, older ones may not).
Your populace page will be your registered SCA name whenever possible.
At the top of the page will be an "Edit" tab. Clicking there will bring you to an edit box with the contents of the page. Here is where all the editing will take place. Wiki mark up takes a little bit to get used to, so feel free to play with it a bit. A good reference of the mark up language is here: Wiki Reference Card
Place a blank line between paragraphs.
Place a * in front of items to create a bullet point list.
The "Show preview" button is your friend! It will show you how the page will look once saved, and if you scroll down to the bottom you'll have your edit box available to keep making changes. This is preferred to saving every little change, since each change is a new entry into the wiki and must be reviewed. If you like what you see, then you can hit "Save page".
We recommend that folks look at what other people have done with their pages to get ideas. You can "edit" someones page to see what the wiki mark up looks like - just be careful not to change and save anything.