Engraving new panel lines are a big plus on a model. Every custom Gundams has a lot of new panel lines, and it really brings more detail to the model. But, I can't add anything, anywhere. I need to be careful with the angles between my lines. If the angle is too narrow or too wild, it won't look realistic. 60 degrees is considered as the perfect angle.
I need to take care of the start of the line, it can't appear out of the blue. The easiest way is to start from an edge. The panel line should give me the idea that it is multiple blocks assembled together to form this part. At least it's the look I want to achieve.
When I have a marked edge, I can draw a line following one or the other edge as a straight line, then at some point, take a 60 degrees angle, keep going, another angle and finish with a line parallel to the first one.
But it's not often the case. So I have to imagine how the block should be separated, and keep an interesting design for the eye. A graphite pen and eraser are my best friends! And also, I need to remember to take pictures of my design! If at some point I erased it, I will be happy to refer to something to get it back (it already happens to me so many times!)