If you search there is plenty of information on the subject.
Add an entry to magictype
Add an entry to server database (which is just magic type in database or flatfile format)
Add handling for skill in source
Add description/icons for skill in client.
It's all quite simple to do. it took me about 5 minutes to figure out just by tinkering with client files.
Really basic skill that I never finished but gives you a good example. This is not replacing any skill and contains its own description and such, sounds, effects, etc (recycling client effects but w/e)