How to what?...
Edit the client? There's a million and a half step by step guides and a ton of fully functional tools to do it for you... why would I help if you wont even search?
Validate data server side? Again... basic common sense and entry level programming. "ooh, they say they are casting the skill at location X/Y.... should we let them do that or is it too far away?" is literally all the check needed to ensure such edits wont effect the server significantly.