This is a video that shows how to create a custom stamp using the Typewriter tool. The basic steps work for most of the tools.
First, tap on the Typewriter icon and tap the document. Type the text you want to appear at the custom stamp.
Once done, tap the stamp icon in the tool ribbon in the upper right hand corner of the app.
Type a name for the stamp and tap the check mark at the end.
That's it! The custom stamp will be available in your stamp library (found in alphabetical order) the next time you access the stamp tool.