We are in the process of creating learning materials for this task, but polishing and publishing them online for everyone will require additional time. If you are ready to test our solution for your own app, I can provide you with draft instructions and test sample in return. It will show how to show a custom control where a user can draw his signature and save the result into an image property within your business class.
Here is what it looks like in action:
For testing you will need XAF v17.2.3+, because there we supported mobile custom modules: Task-Based Help > How to: Add an XAF Mobile Custom Module
If you are interested in this testing, email me at dennis@devexpress.com.