Well, practice makes perfect!
try to stay submerged during the day running at 1knot to save batteries & surface at night, understandingly, the English channel is the hardest to get through, I usually go the other way!
Keep checking the nav map whilst submerged for hydraphone reports of enemy destroyers etc (red line), draw a plot so you know where they are going & keep periscope down, don't be tempted to fire at a destroyer untill you are familiar with SH3, also use the 'Z' key for silent running.
You
will get the hang of things, It's a great game!
Cheers.