Open up the Project you wish an add a signature to. Once on the Summary screen, tap the overflow menu and select the "Add Signature" option. This is also accessible from the Issues list.
Once in the signature screen, tap Add Signature to add your signature (or Edit Signature if you want to change it). If you make a mistake whilst writing your signature, you can use the undo icon in the bottom right to reverse the most recent changes, or you can use the Overflow menu to access an option to clear your signature completely.
From the signature screen you can also enter your name which will be placed underneath your signature on the final report.