Find a doorframe, and put your heel at the base. Slide your leg up so that it is resting on the doorframe. Use your arms to brace yourself during this.
Using your arms, push your body to the frame until you're in the splits. As in regular splits, this won't happen immediately!
Once you can do your splits, try moving your bottom foot out a couple of inches from the doorframe. This increases the stretch.
You can also work on your back flexibility by climbing up the other doorframe, like above.
These may feel more intense than your regular splits because your bottom foot is flexed. Be careful and don't push too hard :) Good luck!