Photograph it flat and bright
Even lighting on white paper gives high contrast, and high contrast survives heavy compression much better than a grey, shadowed scan.
SSC-style signature size
SSC application forms typically ask for a signature in JPG between 10 KB and 20 KB, in a wide strip roughly four times as long as it is tall. Always confirm the exact numbers on the notification for your exam — then match them below.
Keeping a signature readable at small sizes
Crop to the strokes
Every millimetre of blank paper costs bytes. Crop right up to the signature so the size budget is spent on the ink.
Export as JPG
Almost every portal expects JPG. It also reaches small targets with fewer artefacts than PNG for a scanned image.