Typically inscribed twice and on opposite sides of the candle. If you candle is square, you can carve the inscription on all four sides. Once the candle is inscribed it should only be used for that intended purpose.
This is best done within your magickal circle. Especially if this is a candle you will light each night when you come from work and state a prayer over it in a simple lighting ritual. Inscribing your candle inputs your thought, energy and intent into the candle in a focused way. You're basically programming the candle for the intended purpose, and lighting the candle turns on the release of that captured energy for manifestation.
Inscribing is often done with a tool of some sort. If you inscribe a candle you could use your Athame, a pin, a specialist scribing tool or anything else your imagination can dream up. Once you have chosen your tool though, dedicate it properly, then tuck it away in your magickal tool chest and reserve it for just this