Workout Design For Rookies (Part 1): Intent first.
Your intent lays the foundation of a workout before the nuts and bolts can assemble the pieces together. You need to know the why to execute the what. Critical variables ranging from total workload to rest time, exercise selection or order impact the body's response to your training. There's a reason some of the strength and fitness wizards charge the big price for their workouts: Health alchemy is as much art as it is science.
Keep that in mind the next time you do three sets of eight or forty minutes of exertion followed by an hour stretch. Question every detail's purpose to make sure they are in line with your goals.