15. To select the first spine move your pointer to the top left hand
corner of the image. Check that it's right in the corner (look to
the lower left and make sure "x" and "y" are set to "0"). It is often
easier to do this step by "zooming" in on the image (makes
it bigger and easier to work on).

Press and hold down the left mouse button. Drag the
cursor down to the bottom of the image and then move it to the right.
Watch the "x" positioning and get it to "150" (see insets). Release the button.
You now have a selection that is the height of your image and
has a width of 150 pix (1.5cm x 100pix/cm):