The general way is to get a tool called a 'P' cutter which is a curved cutting blade that scrapes a groove. The raised panel lines are sanded to the surface but leaving enough to see where they were. Then, using either a strait edge or 'Dymo' tape as a guide, the 'P' cutter is pulled along the lines until the desired depth is achieved. There are very thin steel flexi template guides available with various holes, round, square and oval in differing sizes. These can be used to get access hatches etc done neatly.
I have tried scribing using pointed tools like dental probes etc but I don't find them that good as they tend to scratch a groove but also raise a lot of the plastic along the edges meaning it has to be sanded back down again, very laborious process. As the 'P' cutter design is to cut a groove rather than scratch it out, a cleaner finish can be achieved.
I nearly bought one of these 'P' cutters at Hinckley show but for some reason I didn't, will have to get one though. Dave and I discussed this very problem at Hinckley and Dave mentioned that he uses the back edge of his blade to achieve the same thing. Having used his tip to scribe the lines in the wood base for my FW190, I can say it does work. However you do it, it is a long laborious job where you have to very gently run whatever tool you use along the line very lightly as many times as required. As soon as you get bored and put pressure on to speed it up, trust me it will all go wrong.
The other thing is that if the raised lines are well done, don't worry about them. I know, although only by seeing kits done with it, that Promodeller Wash (available from Scale Models shop) works with raised panel lines as well.
