The body of an electric guitar is usually made of a thick slab of hardwood. The luthier roughly cuts the intended contours of the body out of this slab, always leaving a safety margin of a few millimetres. The most common tool for cutting the body is the band saw. Other people such as myself don’t own a band saw and make do with a jig saw which works just as well.
From this raw body you work your way towards the final form by removing all material that is outside the planned surface of the guitar. There is a number of tools you can use for this such as chisels, rasps, a spindle sander, but depending on which tool you use it would take ages and be somewhat frustrating, or you would risk overshooting and removing more than you intend. Cutting away stuff is easier than putting it back. To avoid this sort of problem it is easiest to use a template and a router because it gets you as close to the final outline as possible with as little fuss and risk as feasable.
Preparing the template is somewhat time-consuming, but much easier because the template isn’t as thick as the body. Where your typical body blank can be up to 45mm thick, the template would be anywhere between 5 and 15 mm, and you don’t use hardwood for the template but softer plywood or MDF which is much easier to work with.
You come up with a design and draw the intended outline either directly on the template material, or you draw it on paper (I like the semi-transparent paper tailors use for their patterns) and then transfer the pattern from the paper to the template.
Then you cut it out with a jigsaw, again with a few millimetres of safety margin. Then you use a rasp (my favorite is a saw rasp) to file away all material outside your contour line, going ever more careful the closer you get. When you hav almost reached the line, you put the rasp away and use sanding paper to smooth the edges until they are perfect. In the end, the edge should form a smooth, organic line and be perpendicular to the surface of the template.
Sanding a 10 to 15mm thick board of plywood or MDF to a perfectly perpendicular and straight side by hand is a bit difficult – you can’t really move a sanding block freehand without veering from the ideal angle and ending up with a somewhat irregular and rounded side, so I came up with a simple method to ensure a right angle. (Possibly I am not the first woodworker to invent this method, but I haven’t seen it anywhere else yet. Let me know if there are previous descriptions, and I’ll add a link.)
I put a few pieces of very thin plywood onto the workbench and put the template on top of that. Now the edge is a few millimeters above the bench. Now I put sanding paper on the sides of a cork sanding block and move it along the edge I want to smooth over. This way the side of your template will be perfectly perpendicular. You want to make sure the sanding block is thicker than your template so you sand the complete thickness of the template in one go.
For the inside curves of the template, I made a special oval sanding block out of plywood:
I drew the outline on a leftover piece of MDF I happened to have, cut it out with a jigsaw, smoothed it first with the saw rasp and then with a sanding block, then used this template to rout my sanding block out of a piece of 18 mmm plywood (fixed the blank to a lenght of 2by4, used The Trick™ to fix the template on top of it and took the router with a flush trim bit to it. Then you can fix long strips of sanding paper to the outside of the oblong and use that as a sanding block. I added to handles for easier handling.
And for the really tight inside curves, especially where the cutouts meet the fretboard, I made a third sanding block thing out of a length of 19mm round wood, a circular bit of plywood and a length of 8 mm dowel:
Work on the template until it looks good and the sides are nice and straight. If you slide the tips of your fingers along the side (I like to use the index, middle and ring fingers of one hand; using three fingers makes the method more effective because you sense any imperfection three times, not just once, making it less likely to miss something) you will notice where the curve is not right, where there are bumps or depressions that shouldn’t be there. Mark those places with a pencil or crayon and continue working on them with your sanding blocks until they feel perfect. Once you are satisfied that the template is as close to perfect as you want it to be, you’re done.