This is no easy task. Im currently doing this myself, and having to do it manually via code, and no UI program to help with positioning and sizes etc... It's tough. I would recommend only doing this if you have alot of time and patience, and really want the new UI in your server.
Start in MainDialogs.cs as mentioned above. Main Ui at the bottom with HP/MP, and the buttons.Then when that is done, start and work your way through the other Dialogs in the Client/MirScenes/Dialogs folder.
"Remember to backup your source before you start this, as it can seriously mess things up." This is very important!
Goodluck buddy