First, assuming that you're not a beginning lifter, you should determine which muscle groups you will workout for each day that you train (i.e., chest, shoulders, triceps). Once this is known, the next step is to arrange the order of the muscle groups to be trained from largest to smallest muscles (i.e., chest, triceps, shoulders). If you plan on training antagonistic muscle groups (i.e., quads and hamstrings), you should prioritize the muscle you feel needs more strength or size. In this case, for most people, the hamstrings should come first before quads. In other words, you would be wise to train the hamstrings when you have the most energy before hitting quads.
Once the order of muscle groups has been determined, the next step is to arrange the order of exercises. You should prioritize free weight compound exercises first when you have the energy and stamina. For example, you should perform incline dumbbell presses before pec-dec flyes when you work chest. Another example, perform dumbbell military presses before cable lateral raises when you work shoulders. For triceps, you should perform close-grip bench presses before rope pressdowns. On the other hand, if you feel your triceps or shoulders are mostly engaged in the incline dumbbell presses, you should perform a set of pec-dec flyes first to pre-exhaust your chest before hitting incline dumbbell presses.
To sum up, here's a quick general list you should follow to properly arrange your workout program:
- Arrange from largest to smallest muscle groups (i.e., chest before triceps)
- When training antagonistic muscles, train the weakest muscles first (i.e., hamstrings before quads)
- Perform free weight compound exercises before machines and then finish with cable work (i.e., incline dumbbell presses before decline Smith machine presses and finish with cable flyes)
- Perform pre-exhaust isolation exercises before compound exercises when you want to pre-fatigue a particular muscle (i.e., pec-dec flyes before incline dumbbell presses)
Keep in mind that these rules for exercise order are not written in stone. As you become more advanced in your lifting career, you may vary the exercise order based on how your body feels or what you feel is a more effective exercise order. But in general, these are good rules of thumb for you to follow when getting started.