Here are the steps to follow:

1. Sanding: Begin by sanding the surface of the mahogany guitar by hand, finishing with a 220-grit sandpaper. Make sure the sanding is perfect before moving on to the next step.
2. Application of grain filler: Apply one or two coats of mahogany-colored grain filler to fill the wood pores. The color of the grain filler is important as it will not be tinted to enhance the wood grain. After each coat, sand the surface with 320-grit sandpaper.
3. Shellac: Use an orange shellac to seal the wood and bring out the natural color of the mahogany. You can also replace the shellac with an alcohol-based medium mahogany stain to slightly darken the color of the mahogany. Avoid cheap water-based stains as they can mask the wood details.
4. Sealer: Apply two coats of water-based or solvent-based sealer to seal the wood and prepare the surface for the lacquer. Lightly sand the sealer with 400-grit sandpaper between each coat.
5. Lacquer: Apply 7 to 10 coats of glossy water-based or solvent-based lacquer to the surface, lightly sanding with 600-grit sandpaper between each coat. Make sure each coat is completely dry before applying the next one.
6. Polishing: Let the finish dry for 125 hours, then polish the surface to make it shine.

Note that these steps can also be used for other types of wood, but be sure to carefully read the labels of the products you are using and respect the recommended drying times for optimal finishing.