Training a Siamese cat to use a litter box is similar to training any other cat to use a litter box. Here are some steps you can follow to help your Siamese cat learn to use the litter box:

1. Choose a litter box that is the right size for your cat. It should be large enough for your cat to comfortably turn around and move in, but not so large that it is difficult for your cat to reach the litter.
2. Place the litter box in a location that is easily accessible to your cat, but also private and away from their food and water.
3. Fill the litter box with a litter that your cat likes. Some cats prefer clumping litter, while others prefer a more natural litter made from recycled newspaper or wood chips.
4. Show your cat where the litter box is and encourage them to use it by placing them in the box and gently scratching the litter with your hand.
5. Reward your cat with treats and praise when they use the litter box, and be sure to clean the litter box regularly to keep it fresh and inviting.

With patience and consistent training, your Siamese cat should learn to use the litter box in no time.