You can blanch a whole bunch of spinach in boiling water for less than a minute, and it will shrink down to a tiny log that you can smush in your palm! Try to get all the water out. Keep it in this log form so that it will be easy to chop roughly.
You can use premade sauce, but it's just as easy to make your own. Here is what I put in my sauce: rice vinegar, mirin, shoyu, lime juice, sesame oil and grated ginger. It's tangy and fresh. You can toss everything together with the noodles, or you can arrange the spinach and green onion (and surimi) on top for a beautiful presentation, and then drizzle the sauce.