Answer:
For the phrase, "sign language", you add the two words "sign" and "language" together.
Sign: Have your two index fingers extended outwards with rest of your fingers curled in, and move your hands/arms (keep them straight! don't let them flop around!) in alternating circles, which pretty much is the hand movements of a person signing.
Language: Make two "L" shaped figures with your two pointer fingers and thumbs, with the rest of your fingers curled into your palm. Start by touching the tips of your thumbs together, then extend your hands outwards (sideways from one another) letting them flow up and down to look like waves.
Now put the two together and presto! "Sign language!"