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Before the art glass panel goes in, it needs something to support it. You can make a knee wall out of two 3/4" pieces of plywood. It goes flush against the tub and is screwed into the cleats underneath the tub.
Attach hardibacker to the knee wall with 1-1/4" screws.
To make sure the channel is aligned correctly, use a self-leveling laser thumb-bob.
Attach the metal channel with a few screws and then carefully slide the art glass panel into the channel, being very careful not to chip the edges of the glass. Once the glass fits correctly, remove it and put it in a safe place until the end of the project.