Welcome
As a hint you should read the stuff in the stickied "Sub Skipper's Bag [...]" thread. You will be told there what markings on the map become important on which kind of attack method.
I don't wanna rewrite everything here, but I can tell you that you don't need to draw too much. Like when approaching a convoy, you don't need to draw lines and points for every ship you want to attack, of course
. Only one will be sufficient to estimate the convoy's course and speed. The rest can then follow by the use of an AoB-wheel for example.
I just wrote a thread some days ago concerning one attack run on a single freighter of mine. Maybe that's helpful to you, also.
Where was it again ? Ah ... here
http://www.subsim.com/radioroom/showthread.php?t=151917
Have fun, i.