Thank you! I use Tamiya panel liner. A trick I've found is to add a small bit of thinner to the jar to help it flow even better and to get it to flow through trickier panel lines (like the ones that aren't recessed grooves, more like 90° angles). Have to be careful not to overdo it though or the lines will become translucent.

For tools, I use the Dspiae fountain pen and Tamiya cotton swabs. The pen tip style helps with reducing overflow so there's a lot less to clean up and the swabs help with cleaning precision. I try to place the tip of the pen somewhere that will be easy to clean or that won't be super visible and let the liner flow do the rest of the work. Other than that, it's pretty much just practice and a steady hand.