The only way that i know how to get splits down is doing them. If you warm up you legs first (do some jumps or jog or kicks or something) its easier to stretch yourself and it won't be as easy to pull a muscle. do oversplits and sit in your splits.
I once slept in my splits when i was really little. I had them down the next morning but i couldn't walk. I wouldn't suggest this meathod.
What helped me was to sit in my splits while I watched tv (if only I had time to do that anymore), but don't forget to switch off with your splits so you don't end up sitting in one split for 1/2 an hour, ouch!
my mom(whos a yoga teacher) taught me a way to get your splits and what you doo is you go down a far as you can in your splits then get some cussions and pu them under you so that you can realx in your splits it really helps because your musscles dont tense up
try doing oversplits, when you put one leg up on a mat. That really helps me a lot. And do them while you are watching tv as well.
and then, when you get really, really close, do them on your bed. On your bed it will be all the way down, and it is a lot easier to hold it their. Hope this helps