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.
Resize a signature between 10 KB and 20 KB
When a form states a range such as 10 KB to 20 KB, aim for the upper end: the file passes the check and keeps the most detail. The target below is set to 20 KB — lower it if your portal rejects the file.
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.