Drag across the paper to draw. The paper has a grain, and the crayon only catches the high spots, so a light pass leaves gaps. Lines thin out and fade as you move faster, and a pen changes them by pressure as well. Tilt the pen and it paints broad, as if you were using the side of the crayon.
Pick one crayon from the rack in the color panel. The last one in the rack opens any color you like. The tool panel switches between the crayon and the eraser and sets the size, the density, the smoothing, and the response. Smoothing is how much the shake of your hand is evened out, and response is how much pressure and speed show up in the line. The size also moves with [ and ].
The edit panel holds undo, redo, clear all, and save image. Undo and redo also work with Ctrl + Z and Ctrl + Shift + Z.
Every panel is movable. Drag a panel by its header and it snaps to the other panels and to the edges of the screen. Grab an edge to resize it. Double-click the title to switch how the panels are laid out; the fixed layout puts a single panel in the top right.