yes Wazoo's tutorials are essential for Manual TDC. Also this is very helpful:
http://www.subsim.com/radioroom/showthread.php?t=88961
No don't worry about screwing up. You get used to it pretty quick. I had used auto TDC for a while but then I decided to school myself and on the first try I missed so badly I wanted to cry. But on my 2nd attempt I got all the data right and hit a C2 with a 4 torp salvo. Might seem overkill but I didn't want to miss and when all four torps hit the ship didnt go down! I had to put a 5th in her and even then it took a while to list into the ocean. Lots of practice. Practice!
Also to begin with I pause at every step. Get fluid with it and then you're off to the races.