To do the panel lines on this conversion I just broke the tip off a knife and used the back of the knife (number 11 blade), drag it at about 45 degrees and it'll scoop out the material, do it multiple times to create the lines.
Where you snap the knife blade determines the thickness of the line because it tapers, break it closer to the tip for a thinner line. For very fine lines (not the ones on this model) I have used pins (safety pins, sewing pins, different pins give different thicknesses), just dragging them along the surface the same way I do the knife.
Use a ruler to keep the lines straight and don't apply too much pressure to avoid having slips, do multiple passes to get a deep line rather than trying to do it in 1.
I have used the same method to scribe lines in to model aircraft panels (as in, curvy surfaces, not the super-flat lines of a Space Marine thing) but it is significantly tricker.
That said, I'd like to try one of those fancy scribing tools, I'm sure they'd work well too, I've just never gotten around to buying one.