How to properly use manual mode

Hi, so ofc i know how to upshift and down shift and how to put the car into manual mode, but my question is when is it safe for the car to jump into manual mode. Could you do it while already driving regular or should it be done at full stop. Same goes with coming back to automatic after driving manual, could i do it while driving or should i stop completely. Because there are times when im in manual mode and jump back to auto and it jerks the car. Thank you all God bless you