You can modify a master page in exactly the same way as any other page. You can add and delete objects, edit their properties and even their actions.
These properties and actions will appear on all pages in your publication. It is therefore important that the Master Page only contains objects, properties and actions which you want on all your pages and that the position of those objects does not interfere with items on other pages, or vice versa.
Note:
Some of the page templates supplied with some versions of Creator (cover pages, intro pages and dividers) are designed with in-built backgrounds to hide the master page style. Double-clicking on the page will allow you to edit this background and NOT the page itself. To do that you will need to select the page in the Organiser and select Properties from the Edit menu or the right mouse click menu.