First off, I really want to say that the descriptions for these achievements are VERY vague and MISLEADING.
To obtain these achievements, go to Quick play-Exercise Routines-All Exercises- then scroll down to Pushups. There was no set difficulty for this achievement so disregard the difficulty if they ask you.
I feel that it shouldn't have to be said but I REALLY hope you know HOW to do a push-up. If not people will enjoy watching you go for this achievement.
Another very important part that has to be mentioned, while doing said exercise, YOU HAVE TO MATCH the trainer EXACTLY speed, and positioning! If you don't you will not be counted as a perfect one. (Now the tracking may vary, so don't vote negative just because you got it to track you easier without doing as much work)
Now onto the misleading part, if you look in the lower right corner, you will see 5 empty boxes that glow blue (THIS signifies a PERFECT one!). Now as long as you fill up ALL 5 boxes before the routine ends (it usually ends at about 10-15 reps), this will count as ONE set completed. On a side note, once you have all the 5 blue rectangles filled, feel free to stop and save your energy because it will NOT count against you.
Repeat the above routine FOUR more times back-to-back, and you HAVE to get all the blue rectangles filled before the exercise ends for it to count. The only thing that really gets annoying is the tracking, because many times, I would be at 4/5 and then the Kinect sensor would stop tracking me and I would have to start all over.
NOTE: Once the workout ends, click continue and then re-select the same routine and keep going. Once you have all 5 completed, the achievement will pop up as Bob/Jillian finish standing up.
Additionally, I did this achievement two different ways:
1) If you want to do push-ups legit, match Bob/Jillian and complete them all.
2) If you want you can do them on your knees, but make sure that your arms are shoulder-width apart and you are going at the same speed as the trainer.
Good luck! This achievement can take about 10-50 minutes. The time is really based on if the Kinect wants to give you the achievement.