The resting time depends on the temperature, and it was 24°C (75°F) when we made the udon. As a rough guide, rest the dough in a plastic bag for 2 hours in spring and fall, 30 to 60 minutes in summer, and over 3 or 4 hours in winter when you are in Japan.
Use a generous amount of water to cook the udon, otherwise the water gets starchy, and the salt in the noodles doesn’t dissolve into the boiling water.
Corn starch or potato starch helps keep the noodles from sticking. You can substitute flour instead but you should cook them as soon as possible.