Do multiple exercises
The best way to go about building mass on your legs is to do multiple exercises on the same day. You need to push yourself and try and perform as many exercises for your legs as you can. Doing compound exercises which target more than one region in your leg also prove to be amongst the most beneficial to gaining leg mass.
At the same time, while you are working out, go as low as you can on each rep, while looking to complete each rep slowly, which would ensure you focus on your form throughout.
Max out on the weight
Legs are very hard to train and it is even harder to get them to grow big. The only way by which you can ensure a gradual but constant growth in your legs size mass and strength, is if you keep increasing the weight with which you train accordingly.
Going into the gym, you should be looking to increase the weight that you push with your legs week in and week out, which if done properly can result in you having the legs you always dreamed about.
Cut down on cardio training
Considering how you want to get your legs to grow in size, it would be smarter to cut down on the cardio training. Doing cardio would not allow the muscles in your leg to swell up to the level you want.
It would be smarter to cut down the number of days you do cardio training, and to ensure that your leg training days, do not overlap with your cardio days.