1) Make sure your robot is at its "point zero" and ready to draw;
2) Place the markers into the pen holder. They should be distributed clockwise starting from the first marker at top right; when you choose a drawing the app will help you understand the correct position for every marker.
3) The important thing here is precision: if the markers are placed too loosely they won't touch the wall, but if the markers are placed too firmly, the Scribit Pen Holder may get stuck while rotating. This can cause Scribit to draw imprecisely (i.e. adding little lines at the end of a drawing) or to draw with one color only instead of switching between colors.
Newer Scribit units might come with new Scribit Marker Adapter which will help you achieving automatically the correct marker positioning.
These three short videos will explain you the best ways to correctly position your markers inside Scribit.