First download ATAK from the google play store
Click the Hamburger
Select 'Settings'
Choose 'Network Preferences'
Select 'Server Connections'
Click 'Add'
Name - ATXTAK
Address: tak.atxtak.com
(Do not use the server address in the picture)
Check 'use authentication' and enter your user name and password.
Select 'Enroll for Client Certificate'
Click 'OK'
Update v 5.2 - There is an additional box for 'enroll with preconfigured trust' uncheck this box.
Enter User Name and Password Provided.
You may be prompted to accept additional features from the server. These are required packages to turn on certain features and services. Accept these additional software packages for the best experience.
Windows / WinTAK users.
The process is very similar. Navigate to settings and follow the same process.