Effective way to trail other subs?

[Kodiac]

Greetings everyone;

I'm new to DW and looked all over to find my answer and I couldn't find it so, I hope I'm not repeating this topic. I'm want to sneak up to enemy subs, to be able to come up right behind them and they won't know I'm there. I'm not sure what is the best way to approach a sub contact to be not detected and to know exactly what depth they are at without looking at external view? Also the best distance behind the target? Should I have my doors open already before I get there? I guess I would like to experience following other subs for awile before I blow it up.