EDIT:
I have recently tried this method using Warbirds water-based paint and it was not very successful so I would only recommend it for enamel paint. Any other type of paint and I suggest experimenting first.
END OF EDIT
First of all buy a fine tipped permanent marker as near to the colour of the lettering as you can find; it’s amazing how far out the shade can be and still produce good results. Make a card stencil using whatever method you prefer and tape it into position on the model, draw the out line using the permanent marker. Allow plenty of time for the ink to dry then using a fine brush paint to the edges then infill with a “large” brush. The ink “draws” the paint to the edge and results in a smooth outline.
Here's a couple of photos of the registration lettering on my Panall Elf done using this method.
Linear Mode
