7. Now we come to the more precise part of the job.
Move the mouse pointer 150 pix (1.5cm x 100pix/cm) from the left
edge (look in the bottom right hand corner for the "x" position
of the mouse). The "y" position isn't critical but keep it small.

When you are in that position press and hold the shift key down.
With the shift key down press and hold the left mouse button. Drag
the mouse down to the bottom of the image and release.

You will see that you have created a fine vertical line.
We now repeat the above for each spine width (so the next
line will start at 300 pix ("x" position)). We repeat this
6 times (increasing the "x" position by 150 pix from the last line
each time) so we end up with 7 spines.

To finish off use the pen tool to draw a
horizontal line about 150 pix ("y" position) from the bottom
of the image (using the shift and left mouse button combo
we used before). This will finish the guide lines that we will use shortly: