To create a smooth and even look on your stairs, sand everything down before you paint. Stairs should be covered with a sealer and maybe even an old coat of paint. All of this needs to be removed and sanded smooth to avoid odd grooves or a bulky looking paint job. To get into those hard to reach corners, use a piece of sandpaper to manually sand the area. If a few tough spots won't budge, try attaching a piece of sandpaper to the edge and corner of a wooden block to create the 90-degree angle sander you need.
Most people realize that a coat of primer goes a long way when it comes to dark colors, but for maximum effect, use a tinted primer. The color the primer is tinted should be similar to the color you're planning to paint. So, for a cream paint, choose a slight yellow or very light beige or tan tinted primer to help cover the dark color. Primer on wood should be applied in two thin coats instead of one thick one. Thin coats will help to cover the wood grain better than a thick coat, will help you avoid the too-thick paint look when you're done and will highlight any spots you may have missed. Make sure your primer is completely dry before you apply your paint.
Once you've finished painting your stairs with your cream color, seal your stairs with three coats of polyurethane to keep them from becoming scuffed or damaged later. Sealing is especially important with light-colored steps that may become dingy looking if left unsealed. Paint should be completely dry before polyurethane is applied to avoid streaking issues. Although polyurethane does not typically need to dry between coats, allowing the sealer to dry may give you a more durable sealer. Be sure to check the manufacturer's instructions regarding the product you're using to estimate needed drying time.