Select your shelf based on the amount of space it has on the ledge, as well as any decorative brackets or features it might have like sconces or mirrors or additional nooks. While you can use a bookcase shelf and decorate within the cubby-like space, a separate shelf will give you more options.
Choose your paint colors by what makes you feel invigorated and creative. For some this might mean a fiery red or sunny yellow, while others might prefer a cool green or lavender or blue for their peaceful, contemplative atmosphere. There is no wrong answer, just whatever works best for you.
Paint (or stain, if you prefer, for a more natural look) your shelf and allow it to dry completely. You don't have to limit yourself to a single, flat color either. If polka dots make you smile, add a few hand-painted ones or stencil a more intricate pattern if that's more your speed.
Decide what the focal point of your altar will be and give it the place of honor in the center of your shelf. Pictures of artists or artwork you admire can be placed in small purchased frames or construct a frame from folded paper, twigs for a natural look or even small knitting needles, tapestry needles, embroidery scissors or anything else that will make a border.
Arrange the other items you've chosen; the ones that symbolize your art or hobbies or that inspire you to creativity, around the focal point of the altar. While some religious altars focus on representing the elements (earth, air, fire and water) yours does not have to; this is a creative exercise, not necessarily a spiritual one.
Place a candle or two or some incense onto the shelf. For confined spaces--like on a bookshelf or cabinet--lit candles may not be the best idea. You can use an electric candle warmer if the candle is scented or use battery operated tea lights for the effect of a flame without the fire hazard. Incense can be pleasantly stimulating and poses little fire risk since you blow it out right after it catches and let the stick or cone smolder and smoke.
Mount your shelf somewhere in your creative space where you will see it as you work--and be inspired by the items on it--and the memory of the time spent putting it together.