Usually with your fingers. You don't need it. Just enter your Grid Square in your packet. If you want APRS, you can download it for just about any platform or operating system. There are only 4 types of packets.
All packets stand alone and are complete in themselves.
Position, Status, Messages and Other. By definition, everyone has only ONE position and only ONE status at any one time. They may send multiple individual messages but usually messages are only one line. Your objective should be to share the channel with as many other like souls as possible and indicate your participation with one successful packet. Using the Shuttle as a digipeater allows schools and stations to communicate amongst each other while seeing all other stations participating in the event.