[Optional] Traditional Japanese restaurants tie up the noodles to enhance the appearance of the somen and this is how you do it. Tie the edge of somen noodles with cooking twine. This way noodles will stay in one direction while cooking. I normally skip this step for home use.
In a large pot, bring water to a boil. When boiling, remove the wrapper.
Add the somen noodles in the boiling water (do not add salt!). Stir noodles with chopsticks so they don’t stick to each other. Cook according to the package instructions. If necessary, add a little bit of cold water in the pot to prevent overflowing.
Drain somen in a colander and wash the noodles with hands under running water.
Once the noodles are cold, find the knotted parts of somen noodles and pick them up. Cut off the edge and discard. Hold each bundle gently and arrange it nicely on a serving plate. Somen can be served with ice to keep cool.
Cut the scallion finely and grate the ginger. Put them in small dishes.
Pour mentsuyu in individual dipping sauce bowls and add iced water to dilute. I recommend making homemade mentsuyu but if you're too busy, get a bottle of mentsuyu like this. See the instructions. You can check the ratio of mentsuyu to water on this page. This brand recommends 1:3.
Put a small portion of scallion and ginger in the dipping sauce and dip the somen noodles to enjoy! To make it more filling, you can bulk up the noodles by tossing in other ingredients such as shredded egg crepes (see How to Make Kinshi Tamago), julienned cucumbers, boiled okra, and ham.
You can keep the leftovers in an airtight container and store in the refrigerator for up to 2 days. Noodles will be stuck so you may want to loosen up in cold water before serving.