Design symbols for use with Milestones

Is it possible for me to design/create my own special symbols for use in Milestones Pro? If so, can you send a link to an explanation/tutorial?

Yes, Milestones offers a built-in symbol maker which make it possible to design symbols and use them on a Milestones schedule.

It's also possible to use an existing bitmap as a symbol. See: Add a bitmap symbol to the toolbox.