You may create your own pattern, or modify a pattern you have created previously, or modify the built-in pattern called "Test Pattern". You may not change the 4 built in patterns "Dragon", "Kilimanjaro", "Tripoli", or "Towers".
To create a new pattern, choose "Create a Pattern" from the pattern menu. If you wish to modify an already-existing pattern, first pick the pattern you want to modify to make it the current pattern, then choose "Edit Current Pattern" from the pattern menu. Before you create your own pattern, you should modify "Test-Pattern" to get the hang of the pattern editor.
Once you have chosen one of these options, you will be using the Pattern Editor. The screen will be a grid outline, showing the layout of one level of your pattern. Tiles on this level will show as black squares. If you are on a level above level 1 (the bottom level), then the tiles of the previous level will show as a grey outline (see illustration). Now see "Using the Pattern Editor" to find out what to do next.