Hi Guys,
Thanks for the words of confidence but regardless of the degree of decoration, to answer the OP's question, the process of roughing out the lock panels is the same whether you decorate the gun or not. The next step, once that is done varies depending on your objectives and uses tools like what Appalachian shows as well as round files and round scrapers. Much depends on the style you are creating and how tightly radiused you want the molding. Here is an example of an Isaac Haines with moldings cut to a tight radius as Haines did.
Here is an example where that radius is much more subtle and is actually the same as the contour of the stock shaping with a relief cut molding edge dividing it from the rest of the stock.
Here is an example in which the molding is merely a raised bead around the panels.
Much like this original English fowler from the 1760s
And here is an example where there is no cut molding just the general shaping of the wrist and lock area.
Here are some tools I find very useful. The round scrapers are by Fischer and purchased years ago from Brownells but you can easily make them from a flat steel bar.
You are right about thin flat areas around the lock. That is particularly important along the top edge of the lock because you won't have to file a big ugly notch for the flint cock. By keeping the flat thin by the cock and tapering the panel inward to expose the lock bolster where the flint cock comes to rest prevents having to cut a notch
Also keep in mind, the flat does not have to be even all round the lock. It actually looks better if the flat is wider along the bottom of the lock than above. Look at these two examples for comparison. Here is an example that has wide even flats all the way round.
Here they are thin and uneven.
Finally, to keep the edges crisp, do all the shaping, scraping and/or sanding, and then when the stock is about ready for stain, swipe the tops on the lock and side plate panels with 220 or 320 grit sandpaper backed by a flat block of wood.
Good luck,
dave