Do you play modded stock game or TWoS?
In TwoS if you play with Manual TDC, then there is nothing to switch on, you must lock the target with SPACEBAR, enter the solution numbers which are speed, course, bearing, AoB. The TDC will then calculate the necessary Gyro Angle after which you are ready to fire if your sub is reasonably well aligned with that angle (ideally not more than +/- 10° to avoid impact failures).
With the Sjizzle charts you have also a table which you can use for 90° solutions telling you how many degrees from 360° or 180° you should deviate your sub heading to fire torpedo in a straight line (Gyro Angle=0 or 180).
Of course, it's all about having the best solution numbers first, whatever system or tutorial you use
I hope this helps somewhat, I'm not a specialist of Manual TDC or RN, but I know TWoS quite well.