Scan and print, or photocopy your photo (or use tracing paper to make a preliminary tracing), then lightly trace the key points of your image. Take particular care with the ears, eyes and whiskers, and draw as lightly as possible, especially when the outlined area will be white. The idea at this point isn't to make a line drawing of the cat, but to give yourself key reference points - you can easily 'join the dots' freehand. You're looking for edges but more importantly, key changes of tone.
Note that the actual drawing must be very light - this is shown darker so that it will show on-screen.
