Start by seasoning boiling water with salt and boil the mustard seeds for around 30 minutes. If needed add some more water in between, just keep an eye on it.
Once cooked transfer the seeds into a bowl of water and rinse the mustard seeds well to remove the slimy layer. If needed refresh the water once or twice.
Then transfer the seeds onto a tray and spread them. Let them dry completely at 60 °C. This is going to take a couple of hours. Every now and then you need to mix the seeds to prevent them from sticking to each other. When there almost dry you can rub them between your hands, this helps a lot as well.
After that deep fry the seeds at 200 °C for a couple of seconds. Place them in a sieve so you can easily take them out once puffed and crispy. When all the seeds float to the top you know they are ready to be taken out.
Spread them on a tray that’s lined with a kitchen paper and season well with salt. Then let them cool down before you can keep it dry and covered for later.