Since youíve prepped the guitar for the finish, sealed it with grain filler etc, if the finish is now extremely smooth and you only want to darken it, itís time to apply a Toner. Toners can be purchased in rattle cans are easy to apply. The key is to mist spray it, very, very lightly until you get the darker shade youíre looking for. If you donít like the look, just wipe it off and start again. Buy one or two dark Toners and see if you can get the look your after. After thoroughly dry, the instrument will still need a final clear finish. Maybe as many as 4 top coats of clear.
Toners donít soak into the wood. They lay on top like a colored lacquer.
If you don't have time to do it right the first time, when will you have time to do it over?