Start by creating a user-defined palette section. To do that, go to "Edit" -> "Manage Palette", then click "New Section". After that, select your customized nodes, then right-click on one of the selected nodes to bring up the selection context menu and choose "Add to Palette". If you have more than one user-defined palette section, "Add to Palette" will allow you to choose the desired section for the selected elements. (I.e. you can have custom elements in different sections.)