It depends somewhat on how tall you are. If you are over 5'7", you could hold the alto straight in front of you, as you might do standing. Some who are a bit shorter can rest the bottom on the chair between their legs, but I don't suggest that. Shorter people should tip the saxophone off to the right (adjust the mouthpiece accordingly). Whatever you do, don't slump! It is usually wise to sit forward on the chair rather than leaning back, and use a firm chair rather than a soft one.