cp artwork where to start

[G1zm0]

hi after reading and looking lots on here and seeing great cps, i still dont know where to really start, ive got photoshop cs3 and ilistrator, and various others, i boot them up and put in measurements of the cp (in this case 15.3 length, 5.6 height, its a one player cp, with other usual buttons, but how do i put my idea on ink?  where do i get the measurements for buttons ect and place them on cp? i usead a joystick arrow art but it was same size as a4 paper and didnt know how to scale it, any tuts in creating a cp art for begginers, i take it the art needs dont 1st before doing the plexi and holes??