W12 x 152 is a standard structural steel shape, you should be able to find the dimensions for this in the Appendix H your text refers to or just about any steel catalog. Draw it as though it looks like a capital letter "I".
All the second bullet point says is that the dimensions for your foundation plan will go to the center of the "I". Once you've laid out your steel columns in the manner specified in the subsequent bullet points, vertical dimensions will run to the midpoints of the vertical section of the "I", and horizontal dimensions will run to the midpoint of the top (or bottom) of the "I". See below: