You will need plastic containers with lids.
Spread a layer of mango slices and sprinkle with rock salt. Add another layer of mango and sprinkle again with rock salt. Repeat the process until all the slices are salted. This method ensures even salting of the mangoes. Do not stir or toss as this will dislodge the salt between the layers.
Cover your container with a lid. Do not add water!
After 12 hours, the salt would have started pulling the moisture from the mangoes and you will notice there is now liquid in the container. Do not discard this liquid but turn the mangoes over so the brine can also soak the top layer. Cover the container again with a lid.
After another 12 hours, turn the mangoes again and check if the mangoes are pliable by taking a piece of mango and try to bend it in half. If it breaks, the mangoes will need to soak longer.
Check again after several hours. If it doesn’t break when you do the test, you can proceed to the next step.
Drain mangoes well. Do not rinse in water.