Okay, I'll try!

You can also check out this tutorial on BS - / [...] an-s-way-to-render-t35041.html

Thank you guys, I will try now! ^^

The key thing is to use a 2nd layer on top ^^ The lines come out smoother if you use the path tool, but it is difficult to get the hang of. You can always just go "free hand" and just give it a higher weight, which is easier to use (to me, anyway)
but I do suggest you learn to use the path tool that lionobsession suggested, for the best results.
Hope this helped!

Is this for a render? If so, create a new layer above the image. Select a pencil size of 1 or 2 pixels. Then use the path tool to go around the image on any lines you want shown in the render. Make sure you are on your new layer and click Stroke Path. In the next dialog box that comes up select Stroke With a Paint Tool and select the tool to pencil. Click okay and the lines should appear!
I think this is what you mean. If not, just ignore it.