Classical Guitar Purfling:
The classical guitar center purfling strip is usually very conventional, it consists of a 3-part wood detail. The center portion of the wood most often matches the wood purfling around the guitar edges and that wood usually matches the back/side wood material. The inner strips of wood are of contracting wood color, usually a lighter wood like maple is selected.
Acoustic Guitar Purfling:
There are a lot of variations with acoustic guitar center purfling. Some of the options are:
3-piece wood similar to classical guitar
Plastic decorative pattern
Wood decorative pattern
The design that is selected, is up to you or the person who orders a guitar.
Router and router base
2-part epoxy or Titebond
Take the assembled back plate and carefully mark the outline of the instrument by tracing around the template. Center the template shape on the joined back plate. Use a white pencil for dark wood. Chuck up a carbide or diamond bit into a Dremel or Bosch Colt Router and fit it into a router base. Secure the back plate to a plywood work-surface with wood cam clamps
Set a wood fence to guide the router and base. Double and triple check the setting and visually inspect how the bit will contact the wood. Make a first pass, usually right down the center. The cut should be about half the thickness of the back plate.
Move the fence slightly each way to make the channel the correct width. A good way to check this is to make a preliminary cut outside of the back tracing, if there is enough wood. The channel with is correct when you have to use a bit of force to insert the purfling.
Finally apply glue to the purfling and insert it into the purfling channel. Use tightbond glue for domestic woods and 2-part epoxy for exotic woods with resin. The purfling should extend about 1/2″ past the top and bottom edges of the back plate for later trimming. Lightly tap the purfling to seat it all the way in the channel.
Set aside to dry. When dry take a sharp scraper and cut the purfling flush with the back plate.
If the purfling has to be forced into the channel aggressively you risk splitting your center joint of the back.