If you're using a resistance band, keep in mind that one band might not cut it for your entire body. Different muscles have different strengths, so you may want to buy two different resistance bands in different thickness, which determines how difficult they'll be to use. In general, if you're able to complete 8 reps of an exercise using a band, you'll want to select another that provides a greater amount of resistance. 
One of the reasons why I love this specific routine so much is that is it easy for beginners. It does not have you doing a thousand reps and burning you out! Instead, you might notice the reps being a little lower than what you are probably used to and that is because the goal is to gain muscle-not worrying about endurance much. Also, you train 4 days per week-that screams “DO-ABLE” to me! You have two designated days where you work your upper body and the same goes for lower body.

After session 12, consider whether you need to increase the weight for any particular exercise. If you can comfortably do more than the RM of 12 exercises, increase the weight by a modest amount, say two pounds or a kilogram for isolation exercise muscles such as triceps and biceps, and 5 pounds or 2.5.kilograms for compound and large muscle group exercises like squats and deadlifts. When using dumbbells, this would apply to each one. Don’t increase the number of sets beyond 3 at this time.