There are a lot of different techniques out there to create a sketch and everybody uses the one that he/she prefers.
For my sketches I use GIMP (a free program) to recreate the field sketch. The result that you see here is entirely digital.
I use a "eyepiece view" template and add it as a layer on top of the field sketch (after scanning it). On the template I add the stars, thereby keeping the exact same position of the dots on the field sketch. During the observation, I write down notes on the field sketch about brightness, color, shape etc. that will help me to recreate the view as realistically as possible. Total time at the eyepiece is about 90 minutes and another 60 minutes is spent in GIMP.
I hope this explains a bit about the method I use.