Take subway line 5 to Hoehyeon Station, exit 5. Upon exiting, turn right and walk straight for a short distance of 10m or so.
While walking, keep a lookout on your left and spot this blue signboard (as per below) with a tiny entrance.
According to Wikipedia: Kalguksu (literally "knife noodles") is a Korean noodle dish consisting of handmade, knife-cut wheat flour noodles served in a large bowl with broth and other ingredients. It is traditionally considered a seasonal food, consumed most often in summer.