I just finished teaching someone else, so head there for details.
https://miiverse.nintendo.net/posts/AYMHAAACAAADVHkkBvGXZg
The short answer is to use the SPANIM function. For example put this in line 3:
SPANIM, 0, "I", 30, 3108, 30, 3109, 30, 3112, 0
(I put three sprites to show you it can alternate between more than two)
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