To cook crab, follow these simple steps: 1. Bring to boil a liter of water with salt. Use 30g of salt to a liter of water. 2. Once the water is boiled, place the crab into the water for about 10 - 20 minutes. The total amount of cooking time would vary depending on how much crab is being cooked. 3. Once the crab is cooked, remove it from the water and allow it to cool down before using it for your recipe.