I find it easier to work with the image in black and white. So open your image, go to image/decrease colour and chose the 2 color option.
You'll get a box pop up with several options, the settings I use are as the pic but I do sometimes play about to see if an image is improved by using different settings. Click ok when you're happy with the image.
Because the image always opens as a background instead of a layer I always duplicate the layer then delete the background. You wont notice anything different as a rule but it is important.
You now need to chose the magic wand from the side task bar. The magic wand works by chosing linked areas of the same colour.
Click anywhere on the white area and it will all be selected. Hit the delete button and it will disappear.
Your guitar should look like this now. If you look you will see how large a brush I used and how it didnt go over the edge.
Select your 2nd layer and using the magic wand click on the chequered area by the 7, hold the shift key on your keyboard and select the large area that will also be part of the 2nd layer. Go to selections invert and again hit delete go back to selection and invert again. This is where it looks scarey as the entire 2nd guitar has disappeared and you''' be left with the lil dash lines, dont worry. Go to the paint brush tool again and colour it in.