How to create extensions in asterisk-pbx?

A SIP extension is configured in the SIP channel driver configuration file, called sip.conf. (This file resides in the Asterisk configuration directory, which is typically /etc/asterisk.)
Open sip.conf with your favorite text editor, scroll to the bottom of the file, and add a section for your extension. You’ll need to choose your own unique password for each account, and change the permit line to match the settings for your local network.