1) From the Projects screen, tap on the Project you wish to work on to go to the Summary screen for it.
2) Tap on "Add Signature". If a signature is present already, this will say "Edit Signature".
3) On the screen that follows, you will see a preview of any signature in place. Tap on "Add Signature" (or "Edit Signature" if one is already present).
4) Use your finger (or an Apple Pencil, if your device supports this) to draw your signature on the dotted line, then tap "Save" in the top right corner.