first of all, this isn't one of those click bait-y posts that guarantee you'll get your splits in one day. everyone has different levels of natural flexibility, which can determine how long it will take for you to get your splits. these are five stretches that will help you get your left and right splits. (not everyone can do middle splits, so these are only for left and right.)

1) lunge stretch.
for me lunges helped me a lot. you might be thinking of the workout lunges, but the stretch is different (as shown in the image linked below.) going back and forth between bending and straightening your front leg helps too.
https://media1.popsugar-assets.com/files/2014/05/06/789/n/1922729/82177076062d0211_move-to-warm-up.xxxlarge.jpg

2) pigeon stretch.
the pigeon stretch is like a flat split but with a bent front leg. try to parallel your calf to the wall in front of you for a deeper stretch.
http://speedendurance.com/wp-content/uploads/2016/02/Floor-Pigeon-Stretch-Pose.jpg

3) lunge quad stretch.
i'm not sure what this stretch is officially called, but as you can see linked below this stretch is like a lunge stretch, but with a dropped and bent back knee. try pulling your back foot further towards your body for a deeper stretch.
http://images.shape.mdpcdn.com/sites/shape.com/files/styles/slide/public/media/crescent-lunge-with-quad-stretch-700_0.jpg

4) try holding your splits.
this one seems obvious, but i feel like it goes overlooked sometimes. for me, i didn't like doing the splits until i got them. but really you can hold your splits while doing other things like watching tv. just make sure that your front leg is straight no matter how far down you are.

this next one is for when you get your splits and don't feel a stretch anymore.

5) oversplits.
oversplits are when you do the splits with your front and/or back leg rested on a raised platform like a mat, a book, a stool, or anything like that.
http://68.media.tumblr.com/tumblr_lx5ibnX2SM1qkmiv3.jpg

thanks for reading and good luck!