Previewing is a good first step.
I can pretty much tell what moves will be challenging from a preview, then I pay special attention to them, breaking them down into their 8-count if necessary (it's standard in the aerobics industry to break moves down into 8-count moves,chaining them together to make 16- or 32-count combos) and paying attention to what foot does what. In Body Max 2, for example, the "speed skater" threw me at first, and I would end up exiting off the wrong side of the step, until I counted it out, and figured that count 6 is a knee up, with counts 7 and 8 exiting off that same side.
Also pay attention to transitions from one side of the step to another (like side to back or back to side) as there are sometimes moves that work alright if you make sure to place your feet correctly to prepare them for the following move, but can be 'torquey if you don't pay attention to that.
Also, sometimes Cathe's cuing of a move (or name for it) doesn't quite make sense to me, so I rename it or 'recue' it to make it easier to me. For example, the triple around the step is usually cued as 'knee up, triple, triple,' but that can throw me, because there are two exit steps before the triples. I cue it to myself as 'knee up-step-step-triple-triple.' And in Low Max(?) Cathe calls a move a "knee-smash repeater" (which logically should be three knee smashes, but is a knee-kick-knee--and which she calls, more logically, a 'kickbox repeater' in Cardio and Weights), but I 'call it" a 'kickbox repeater' or a 'knee-kick-knee.'
Also, sometimes if you find a move difficult, it may help to watch Cedie or another background exerciser, who may do it a bit differently. For example, in Step Blast, I had a bit of trouble with a step that goes off the side and pivots and returns to the step, until I paid attention to how Cedie does it: her move is more circular than how the others do it, and following her lead made it easier for me to end up where I was supposed to.
HTH