Even if you're painting something that stays indoors, you'll want to paint it outside because the outdoors is well-ventilated. Painting outdoors will also help to avoid getting paint anywhere you don't want it. Pick a warm, dry day to tackle this project. Avoid painting when surfaces are damp, when the humidity is more than 80 percent, if the temperature is less than 50 degrees or if the object you're painting is directly in the sun.
Depending on what you're painting and whether you can move it, choose an open area and cover the ground with a large drop cloth or tarp to create your workspace. If you're painting something that's immobile, such as a fence or railing, just make sure that the ground and surrounding surfaces are protected by drop cloths so they aren't marred by the paint if the overspray is caught by the wind.
For railings and fences, consider setting up a large piece of cardboard, lightweight wood, or hanging a drop cloth or sheet as a shield against overspray that might blow onto other objects. Place the "shield" on the side opposite of the one that you're painting.