It is common practice in cardigan patterns to write out shaping for only one front and then indicate that the knitter or crocheter is to "reverse shaping" for the second front. The practice developed because in print, leaving out those instructions means that there is room for something else (like an additional pattern or a more extensive explanation of a stitch pattern or another picture of the garment) and leaves less room for error (in terms of conflicting information or directions). Some people are bothered by this , but I actually really like that it gives me a chance to see what the structure of the garment is rather than just blindly following along with the pattern. Regardless, there are really only two ways to efficiently and effectively reverse shaping, and which method is appropriate will depend on the particular pattern.
The simplest way to reverse shaping is to simply shift all the shaping a single row. So if you were originally working the shaping on the RS rows, for the other front you will work it on the WS rows. For example, "Row 1: k24, k2tog; Row 2: p across" would simply become: "Row 1: k across; Row 2: p24, p2tog". Note that the stitches and decreases are worked in the same order using this method--all you're doing is shifting rows. (Crocheters, this is the same for you--just substitute, for instance, "sc" instead of "k" and "p".) You'll end up with an extra row on one side or the other, but unless you are working at an exceptionally large gauge, this won't make a difference once you've got the sweater put together.
The more complicated method involves actually reversing the rows. In the example above, you would still have "Row 1: k24, k2tog; Row 2: p across" for your first front, but then for the second front you would work ""Row 1: k2tog, k24; Row 2: p across". This method is most useful when you're doing something that involves a lot of patterning, where shifting the decreases would result in a noticeable difference or in the situation noted above, when you're working at an exceptionally large gauge. If you're going to follow this method, I strongly recommend that you actually write out each shaping row reversed before you start working.
A couple of other things to think about: Remember that if you are working in a reversible stitch pattern, like garter or seed stitch, there is no need to actually reverse the shaping. Just make two fronts exactly the same and flip one over! Also, if you're finding something confusing or something doesn't seem right, take a moment to sit and think about it...often there will be a common sense, logical answer that will straighten everything out. For instance, if you've bound off for the armhole at one edge and now you're finding that what should be the neck decreases are also on that edge, something has obviously gone wrong and you need to take a moment to go back and review what's going on with your work. Usually a quick check against the original shaping instructions will get you back on track, and you'll be well on your way to have two mirror-image fronts for your cardigan.