As Cammy, when should I be using crouching light punch into crouching medium punch?

Hello, I'm relatively new to fighting games (currently Plat) and I've seen this string in a couple replays. I tried practicing it in training mode but it doesn't seem like it goes together when I have the dummy on block after first hit. When should I be using this? The combo I'm practicing is Crouching LP -> Crouching MP -> DR -> Back Mp -> Back Mp -> Back HK -> MP (air) -> Cannon strike. Thank you!