:-) There are many ways, but it really depends on what you have to do the job. The ideal tool is a scriber made for the job, such as the Olfa P-cutter, or the Bare Metal Scriber. These actually remove a hair of plastic. However you can also use something as simple as a needle chucked in a pin vice (I also have many home made scribers). If using a needle you will have to sand, lightly, afterwards. (It may also be beneficial to sharpen the needle, using emery paper or a stone). You also ought to use a straight edge, on flat surfaces a steel riler will do, but when scribing over the compound curves of the fuselage you need a straight edge that will conform. A lot of modellers recomend Dymo-Tape. It's advantage is that it has it's own adhesive and will stick to the model surface. It's disadvantage is that it is stiff, and is reluvtant to conform. One medium I have had success with is the thick aluminium foil you get, sealing large tins of instant coffee. Cut a strip, using a hobby knife and a steel rule, about 3-5mm wide (the more compound the curve the thinner it needs to be, generally) Lay a piece of tape along it, extending over one side, use the other side to scribe against. Be gentle at first , otherwise, if you push to hard against the foil it will distort. Once you have made an indentation, in the plastic, this becomes your guid. Take it easy and don't scribe to deep. HTH :-)
Mal