Rinse the kidney with water. Put on gloves if you choose to. Cut off any parts that connected to the kidney. You might still see some of the membranes covering the kidney, gently pull on that and remove it
Use a sharp knife to cut the kidney into half lengthwise. You will see all the whitish fats and veins. All those need to be removed. Use a sharp knife to slice off those fats and veins. Use kitchen shears to cut off some stubborn fats. It's important to remove all the white part to prevent odor when you cook the kidney
Preparing the kidney:
Use a small knife to score the smooth surface of the kidney with criss-cross patterns, taking care not to cut through it. Cut into medium to large slices for cooking
Soak the kidney in saltwater or in milk:
Place these in a bowl of cold water with 1 tsp of salt, enough to cover the kidney and let it sit in the fridge for at least 2 hours or overnight if you can
You can also pour cold milk (without salt), enough to cover and soak the kidneys. Soak for at least 2 hours or overnight if you can
If you plan to soak overnight, change the saltwater or milk before you head to bed for best result
Drain off the soaking liquid and rinse the kidney with fresh water. The kidney is pretty much ready to be used at this point
Parboiled the kidney:
Bring water to a rolling boil and then cook them for about 30 seconds (depending on how big of a slice), they should be gray in color but not fully cooked. This step is to get rid of the blood scums in your cooking. Do not overcook them or they will turn tough
If you choose to store them:
Freeze the parboiled kidney once they cooled down if you do not plan to use them right away. They can be kept frozen for up to 1 month for best result