How do to fly indoors with the use of the sonar?

Eduardo,
As you know, if you're inside, your likelihood of gaining GPS signal is somewhere between slim and none.
On your controller, select the toggle on the top left (Flight Mode Switch) to "P"
Place your aircraft on a flat surface.
The vision positioning system comes on automatically when your Inspire 1 powers up.
See page 16 on the Inspire 1 User Manual
http://download.dji-innovations.com/downloads/inspire_1/en/Inspire_1_User_Manual_v1.0_en.pdf

Cheers!