Use drawingskills to improve your drawing accuracy. Marks that match the model show faintly green. A slight miss turns yellow; a larger miss turns orange. Upload your reference, which will be reduced to its contours.
Make your drawing, then point the camera at your paper, and corrections will show.
Point the camera at your paper and trace the ghost with your pencil. The camera must be still and pointed perpendicularly to the paper, to avoid distortions.
The ghost is lined up with your drawing automatically. Tap Best fit to place it where the most of your drawing turns green (at the current strictness); it shows the green % it found. Then use Adjust ghost to drag, pinch or twist it by hand if you like. Reset goes back to the automatic placement. In full screen the same controls are in the bar at the bottom: Adjust ghost, Best fit, Nudge (small step buttons) and Reset.
Each reference you upload is reduced to contours automatically, which costs 1 credit. If the image model refuses the picture (often figure or nude studies), a free local line drawing is used instead and no credit is spent.