From BlenderWiki

Preparing

The empty Grease Pencil floating panel.

Now all options are visible.

1. The first step when using Grease Pencil is to enable the display of Grease Pencil drawings for the relevant view. To do this, locate the Grease Pencil... entry in the View menu, and then click on the Use Grease Pencil toggle, in the Grease Pencil floating panel that appears.
2. At this point, click on Add New Layer to add a new layer to draw on. This step is not necessary when you are starting off a new drawing (as a new layer will automatically be created for you), unless you want to customize the line width, color, and opacity, before drawing. However, if you want to draw on a new layer once layers already exist, it is necessary to click on the button.

Grease Pencil is available for the following window types: 3D View, Node Editor, UV/Image Editor, and Video Sequence Editor.

Drawing

Quick Usage (For Just A Few Strokes)

• To draw a stroke: While holding ⇧ ShiftLMB , start dragging the mouse to draw a new stroke. The stroke will finish when you release the mouse button.
• To erase stroke(s): While holding AltRMB , start dragging the mouse to erase segments of strokes that fall within the radius of the eraser “brush”.

Note for GNU/Linux users Under some window managers (like Metacity, the default Gnome one), the Alt key is by default used to move/display the main window “menu”. Which means that you won’t be able to erase strokes with AltRMB dragging. You have two solutions: either to enable Draw Mode (see below), or to modify your Gnome settings (e.g. by using the “Super”/“Windows” key instead of the Alt one, in general, in something like the System » Preferences » Windows menu).

Easier usage (for drawing more complex sketches)

1. Enable the Draw Mode toggle in top right-hand corner of the Grease Pencil panel.
2. As for quickly drawing a few strokes, use the same mouse buttons to draw and erase, but without needing to use the modifier keys too (i.e. LMB to draw, RMB to erase).

Special Tricks in “Draw Mode”

• Drawing a straight line: Hold Ctrl while dragging with the LMB to draw a straight line. Although a wavy line will still appear on screen, only the endpoints of that stroke will be used for the final stroke that gets stored. This is a useful feature for architectural uses.
• Drawing a dot: Simply click on a spot. This is mentioned here because it is not available when Draw Mode is not enabled.

For Tablet Users

• The thickness of a stroke at a particular point is affected by the pressure used when drawing that part of the stroke.
• The “eraser” end of the stylus can be used to erase strokes too.

Sensitivity When Drawing

The default settings for the sensitivity to mouse/stylus movement when drawing, have been set so that there shouldn’t be too much jitter while still allowing for fine details to be made. However, sometimes these settings may not be appropriate, in which case, the defaults can be found in the User Preferences window, in Edit Methods “panel”.

• Manhatten Distance: The minimum number of pixels the mouse should have moved either horizontally or vertically before the movement is recorded. Decreasing this should work better for curvy lines.
• Euclidean Distance: The minimum distance that mouse should have traveled before movement is recorded. Imagine this as length of imaginary string between last recorded position and mouse-cursor.
• Eraser Radius: This is self-explanatory. It is simply the size of the eraser “brush”, so changing this will affect how likely strokes are going to be covered within the eraser brush and thus erased.
• Smooth Stroke: This turns on the post-processing step of smoothing the stroke to remove jitter. It is only relevant when not drawing straight lines. By default this is off, as it can often cause “shrinking” of drawings, which is sometimes not that desirable.

Additional notes

• When Draw Mode is enabled, many of the other events that are attached to the LMB and RMB are blocked.
• If you have swapped mouse buttons (in User Preferences window, View & Controls panel, Select with: buttons block), this has no effect on the mapping of mouse-buttons to drawing/erasing operations. However, it may become difficult to select using ⇧ ShiftLMB in such a situation, in which case the tiny “lock” icon beside the Draw Mode button should be enabled to help alleviate the problems (that will simply disable drawing from occurring with ⇧ ShiftLMB ).

Drawing Planes

Sketches are only relevant for the view/view-angle (referred to here as the “drawing plane”) that they were drawn at. There are several different options for how individual strokes (determined by the settings in use when the stroke was created) will be displayed.

• Screen-Aligned: This is the default drawing plane for the 3D view, and is also the viewing plane that gets used for the other editors when Stick to View is disabled. All new strokes that get drawn in this mode appear to be “stuck” to the screen-surface (much like markers on a monitor), and will therefore stay unaffected by zooming/translating/rotating the view.
• View aligned (default for all 2D Views): New strokes are affected by manipulating the view. This can be turned on/off using the Stick to View option.
• Drawing in 3D-Space (only available in the 3D view): When Sketch in 3D is enabled, new strokes are drawn in 3D-space, with the position of their points being determined by the position of the 3D-cursor and the view rotation at the time (i.e. they are on the plane orthogonal to the view direction, and passing through the 3D cursor).