In partitionguessr, you create an OpenStreetMap Overpass Turbo query. The returned points are joined into a short path and plotted as your partition line. With a long enough query you could effectively gerrymander anything, so the query is limited to 256 characters.
Aim for a 50% / 50% split of points on each side of your line (0% difference). If your split is 55% / 45%, your score is 5% difference.
Skill grades:
< 5% GOLD
< 10% SILVER
< 15% BRONZE