It’s really simply!
In Gallery, select an image and press “Share” / “Add Watermark”.
Or start “Add Watermark” and select a source image you want to modify:
You will see the loaded image in preview area with a sample predefined watermark.
If needed rotate the source image:
Press “Settings” or long press on watermark:
Make sure that watermark type selected “as text” (tab “General”):
Then adjust the text and its properties (tab “Text”):
Press “Back” to return to the main screen and adjust size and transparency of the watermark:
Place watermark with your finger or use alignment dialog:
Finally press “Save” to save the image:
Link to install Add Watermark Free from your Android phone.
Link to the web-version of Google Play