People tend to ignore Legs workout from their workout routine as they are not apparently visible from outside. But do remember that legs form the base of human body. Not for nothing did Albert Einstein once said - “The legs are the wheels of creativity". I still do not understand this quote completely though :)..
Legs  probably form the most important body part of a human body and ignoring these precious gifts will not do you any good. You do not want to end up with great chest or arms but skinny chicken legs.You need strong legs to support your strong upper body else you would end up with a hunched physique.
Working out legs is not that time consuming too. You can finish your legs exercise withing half an hour max. You can combine this with any of your upper body workouts too.I prefer combining back and legs in a day.Refer the Workout routines to see the list of leg exercises in combination with back workout. Go for compound exercises when you work out your legs like Squats or Lunges..
Keep following this blog and the website.I will be adding a few videos on how to do these exercises with correct posture.
 

HIIT or High Intensity Interval Training-
Note: HIIT is intended only for advanced trainers. Beginners should stick to the normal 30 minutes cardio workout.
Steps-
1. Strech-Strech your muscles properly specifically the leg muscles.
2. WarmUp- Warmup by doing brisk walkin for a couple of minutes
3. Get Set- Now start running at your normal speed for a period of 6 minutes.
4. Sprint- Now speed the treadmill to the fastest speed you can run.Basically you need to sprint now for a period of 1 minute.
5. Slow down- Now get back to your original speed and keep running for a furthur 5 minutes.
6. Sprint- Repeat step 4.
7. Repeat step 5.
8. CoolDown-Repeat step 2.

This should take you a total of 20-25 minutes.Once you get comfortable with this routine you can reduce the slowdown period and increase the sprint periods.
As this is a high intensity routine, do not do this daily. 2-3 times a week in just fine.