The same way that crawling builds strength though the hips and shoulders through pressure, (think planking for babies) this simple exercise builds strength in your baby’s legs to prepare him for walking.
Have your baby straddle your leg or sit on your knee with both of his legs in front of him. You can also sit 
him in a small chair, but the goal is to have his hips, knees, and ankles each at 90 degrees and usually sitting on your thigh works well for this.
Apply gentle pressure on his thighs, pushing down through his knees, ankles, and feet. This pressure helps to strengthen the joints and improve your baby’s ability to pull to stand and take those precious first steps. Do this 2 or 3 times per day for 6-8 repetitions and hold for 4-5 seconds.
References:
Danto, A. & Pruzansky, M. (2011). 1001 Pediatric Treatment Activities. Creative Ideas for Therapy Sessions. Slack Incorporated.