An alternate way of setting the output of a key is clicking it. The default behaviour is for the dialog to open when you double-click a key in the Ukelele window, but this can be set to a single click in the preferences.

When you double-click (or click) a key, a dialog will open, showing you the current output, and allowing you to enter new output. This can be done using such sources as the character palette, or you can use numeric notation for Unicode code points.

You can have up to twenty characters produced by a single key. If you try to specify something longer, Ukelele will warn you that it cannot do so, and ask you to enter the output again.