To sign along, there are several alternatives. If you mean, "along the length of time", use your two index fingers to draw a parallel path from your body to about a foot in front of you, keeping your fists parallel in the process.
If you mean, "get along with" siblings or friends, etc., move your two curved, open hands a short distance away from your body (with thumbs sticking out while the rest of the fingers are held together on each hand, while the fingertips of both curved hands are facing each other but not touching). Finally, if you mean "go along with", form your two fists with thumbs sticking out and hold them together as they move forward.
Teach your child the along sign in the context of story telling, explaining that an activity is in progress ("moving along"), or going somewhere in pairs or groups ("coming/going along with").