A/D or left/right arrow to walk
right mouse button, X or up arrow to jump
S or down arrow to crouch
while crouching press S or down arrow to fall off cars, scaffoldings or ledges
stand before the blue marked canvases and press space
now you can choose between the graffiti pieces by pressing A/D, left/right arrow, Q/E or Z/C
press space again to choose the piece or press right mouse button or X to abort
stand before the piece and hold space to paint until the piece is finished
You can also choose between pieces while waiting or walking by pressing Q/E or Z/C.
This is useful since it saves time.
The game saves automatically every time you enter the map.
Check out the tutorial level for further instructions.