To allow the guest to sign their photo using a touch screen or a mouse first design the print template with a defined signing area.
In the template editor click on the Signature icon in the lower right (see item 1 below) then resize and place the area where you want the signature to go. (see item 2 below)
In the dialog you can set the Pen color (item 3 below) the pen width (item 4 below) and the background color and opacity (item 5 below). 0 opacity is transparent and 100 is solid.
Last check Photo Signature and enter any text prompt you want.