There are a number of options available, including:
- localised wash using thinned paint
- very fine point marking pen
- draftsman's pen
- draftsman's ink bow
With the two latter options, if you use Indian ink to fill the lines, if there is any overflow or uneven application, you can simply use a damp paper towel to remove the excess (assuming that it's been applied to a glossy surface - if applied to a flat surface it may leach into the surface and will be difficult to remove)