Making a mood theme is simple. First you have to think about the size of your mood theme icons (I prefer width 250 height 83 pixels) this is up to you.
When you save your icons best use the mood theme moods like energetic etc. so you can later upload the correct icons to your mood theme manager (http://www.livejournal.com/manage/moodthemes.bml).
First create a new mood theme with the name of your choosing and then you can edit it by uploading your mood theme icons to photobucket and copying the URL to the manager.