It can be complex it just depend on the content you'd like in it.

Here's a basic example

<?xml version="1.0" encoding="utf-8" ?>

<j:jelly trim="false" xmlns:j="jelly:core" xmlns:g="glide" xmlns:j2="null" xmlns:g2="null">

<div style='width:128px;' align="center">

<center>

<img src="http://preloaders.net/preloaders/464/Smile-128.gif" id="image" height="128px" width="128px" align="center"/>

</center>

</div>

</j:jelly>

but you can find more help in these links:

http://www.servicenowguru.com/system-ui/glidedialogwindow-advanced-popups-ui-pages/

http://www.servicenowguru.com/system-ui/glidedialogwindow_popup_record_list/comment-page-1/

http://wiki.servicenow.com/index.php?title=UI_Pages#gsc.tab=0

This one is great for how to use standard field types in a ui page and explains about the 2 tier loading that happens with jelly

http://snaug.com/?page_id=39