How do I signal a successful kick at goal?

One assistant referee or touch judge stands at or behind each goal post. If the ball goes over the crossbar and between the posts, you raise the flags to indicate a goal (Law 6.25). While judging a kick at goal you stand in in-goal, behind the goal posts (Law 6.22).

The match official signals describe the same signal as "Kick at goal successful": raise the flag to indicate that the ball has gone over the crossbar and between the posts.