Start with the aorta. What are the 3 main branches? Trace the blood flow from one branch until the end of its path. The do the 2nd one, then the 3rd. Follow it all the way back to the heart via the veins and you've got your complete loop.
Like I said in my previous post, this is easier if you draw it as a rudimentary stick figure. Draw a heart with the 3 main branch coming off the aorta and sketch accordingly. It doesn't have to be proportional, just as long as you get the general concept. It will firmly entrench the correct path of blood flow if you draw it a few times.