TouchOSC layouts can be created and modified with the free TouchOSC Editor application for OS X/Windows/Linux which you can download here. Note that the editor application is written in Java and needs a Java run-time, which you can download and install from the java.com website.
When you run the editor application you'll see the window pictured above. At the top you'll find the editing toolbar, to the right is the editing view with a preview of the layout and to left the properties panel which displays contextual information and properties of whatever object is currently selected in the editing view. In the following sections we'll go over all the editor application's functionality.