Rickson Gracie teaches in the following video invisible details to do a double-leg takedown. 

The first important detail the master shows is to keep your shoulder connected to your neck to take your training partner down. This way, they cannot attack your neck. 

The second crucial detail is to avoid lifting and pushing your opponent forward because if they sprawl, you don't get the position. To take your training partner down, even if they sprawl, you must pull them using your arms and hips.     

Afterward, the master will use a wall to help his students understand the connection they need to finish the takedown. 

Watch Rickson's students practicing the position to learn how you can practice it with your training partners.

Keep your eyes and ears open for all the details. Keep training hard!