You can find scribing tools at your better Hobby Shops....squadron makes one that I use.
After you practice on spare plastic parts you will get the hang of it. There are a few modelers tricks for hand scribing lines but the best way is to use a tool meant for the job. A good scriber tool 'removes' the plastic as you pull back.....other methods just plow the material to the sides and you do not want that!
You might also look for some of the old labeling tape at your local office supply store. You know, the plastic, old-fashioned type stuff. You can cut it to length, and use it as a guide for your scriber. Helps to ensure straight lines around curves and odd angles.
Also this is what I did when I had LOTS of lines to scribe....
Here I am with an OLD Aurora 1/48 scale Chinook helicopter from 1963 or 64 ....anyway....It needed panel lines in a serious way.....lots of work!
SO....to keep my panel lines nice and straight I used some thin sheet aluminum ( you can use a empty DR. PEPPER can or coke ) took about a 3"X3" piece, folded it and used it as a guide!.....that way you can create slight curves that will match your items shape!
I've done my share of scratchbuilding over the years, often a car body surface being in dissimilar materials (styrene & resin, with a bit of putty), and when I do something in that manner, I find scribers don't always work that well to make a consistent panel line.
Quite by accident, I discovered that the simple razor saw blade, if drawn backstroke across the surface, will do so very predictably, and can be used to create a uniformly straight, or even slightly curved panel (or in my case, door, hood or trunk) lines quite easily. I have even gone so far as to take an Xacto razor saw blade, and with my Dremel & a carbide cutter, carved the back edge of the blade into a slight "hook" with just the last few teeth of the sawblade sort of "hanging in midair". I draw the lines on the body shell, and then, finding where those lines cross a convex surface, start the panel line with the saw, on the backstroke, by pushing it backwards until I get the groove (panel line) started, then simply extend that line to its corners, where I work the curved corners carefully with the teeth at the rear tip of the blade. It takes a bit of time, and on occasion a tiny bit of putty to fill any extraneous scratches along the edge of the grooved panel line, but it does work well. Of course, the door line on a 1:25 scale car body is wider and deeper than a panel line on an aircraft, but the technique I use does work.
For small access panels, such as a gas door on the body, I like the dozen or so different PE templates I've picked up over the years, generally I can select a shape from one of these that works, often exactly the shape and dimension I want--for these I use the BMF panel scriber, which also works great.