1. Choose your timing intervals
Part of this decision rests on what type of circuit you want to do, and what your goals are. If you are just beginning, an aerobic circuit with a work to rest ratio of 1:1 is appropriate. The means that the work and recovery times are equal and somewhat steady state. This type of circuit can also be done where you change exercises every couple minutes between strength and cardio working at about a 5-7 on a 1-10 intensity scale.
If you are looking for more of a challenge and more calorie burn, you might try an anaerobic circuit to take your fitness to the next level. If you change the work to rest ratio to 1:3 the overall work time should decrease to allow for an increase in intensity (shorter and harder). For example you might exercise for 15 seconds HARD, then rest 45 seconds before repeating the exercise again.
Finally, if you are seeking improvements in performance as an athlete, using the standard Tabata intervals is a good option. This is were you exercise for 20 seconds at a near MAX level and rest only 10 seconds. This type of circuit will push you and improve your oxygen uptake greatly.
2. Choose the type of exercises
Circuits are good time-saving type of workouts, no matter how you choose to design them. You could do an all cardio, all strength or combination of the two in your routine. You can also choose to do all body weight exercises, which is a great choice for at-home workouts. Again, choosing the exercises to fit your goals is important and also to provide variety in your workouts. Remember that form and control ALWAYS trumps speed of exercise. As you fatigue, you must be sure to concentrate on maintaining good form so that you do not get injured.
3. Choose the number of rounds
How much time you have, what equipment is available and your current fitness level are all variables that will determine how many rounds you complete. For example, if you have a 5 exercise circuit that takes you 5 minutes to complete (15 sec work/45 sec rest), you could complete that 4 times with a 5 minute warm-up and 5 minute cool-down to get a kick butt 30 minute workout! To get the most out of whatever circuit workout you do, be sure to work hard during the “work” periods and rest during your recovery times. This will ensure that you get great results from your workouts!
Here's a good example of a typical circuit workout from the Matz Fitness app. You could do these exercises for time or reps or a combination depending on the exercise. For example, jump rope could be timed for 30 seconds and band squat press for 10 reps. Allow yourself 30 seconds or whatever feels right for you between each exercise, and complete five rounds.