1) The strap can slide within the bracket so you can try it with the buckle part in different positions to suit your stroller tube. This video explains https://rockitrocker.com/#instruction-video 

2) You can fit it to the padded handle of your stroller to help get a better grip.

3) Mounting higher on the stroller handle gives the best rock, as it has more leverage.

It's worth noting that battery life is best when the Rockit is vertical, or as close to vertical as possible (pointing up with the button on top). It can be mounted horizontally, but when it is flat like this it will drain the batteries faster.