I've chosen the backskirt part of the MG Zaku Cannon for this quick tutorial. Here's what you're going to need:


X-Acto Knife, Scribing tool, Compass, Dymo Tape, Mr. Surfacer 500, Your Gundam Part

The Zaku part itself:



The first thing you'll have to do is take an ordinary needle and cut it in half. Then replace the blade in the knife with the shortened needle. The needle has to be cut in half because it would otherwise bend and brake if too much pressure is applied




You can now use the needle knife to make a point of orientation on the part. This can be placed anywhere you wish, as long as it is the end of one panel line. It will help you stop scribing the line at a predetermined point.




Next, cut off a piece of Dymo Tape and use it as a template for the panel line you will scribe. Make sure it is positioned directly under the point of orientation.



Now you can start by scribing the line with the needle knife, do not apply pressure! Just move from left to right. You can repeat this as often as you like. Stop once you've got a noticeable panel line. Do not remove the tape.



Now, repeat this process with the scribing tool. Once again, don't apply pressure or you will slip and scratch the part. Repeat until you have your final panel line. You can now remove the tape.




To continue the panel line off to the left, we will place another piece of Dymo Tape on the part, once again it sits right below the point of orientation.



Repeat the procedure mentioned above and you will have something that looks like this:



Now you have to clean up the area where the two panel lines meet. Use the scribing tool to do this and it will look like this:



You should now sand the part with at least 800 grit sand paper to make everything smooth. Remove the plastic dust that gets inside the scribed lines with the needle knife. Notice the scratch at the bottom that I totally made accidentally (cough). We'll address that later.

Now we will focus on copying the panel line to the right side. For this you will need a compass. We will use it to determine the exact location of the point of orientation on the other side of the part.
Here I am measuring the distance from the already present point of orientation to an already present bit of detail



It is copied to the other side.



Now we need a second line from the compass. This is determined by measuring the length of the distance between the point of orientation and the detail on the other side.



Now we can determine the exact position of the POO XD



Same procedure as earlier. Use the needle knife.



Just repeat the entire procedure mentioned above and you'll have matching panel lines. As for the scratch on the left side:



Use Mr. Surfacer 500 to fill that and let it dry. Once it has dried sand the excess Surfacer away with 800 grit sand paper. If any Surfacer gets into the panel line, use the scribing tools to remove it.



Have fun!