You download and install TDC Erhverv SkyApp from App Store, if you use iPhone, or Google Play, if you use Android. The app is free.
Search for "tdc erhverv skyapp".
To sign in you will need your username, which is your work e-mail, and a password. If you do not know your password or forgot it, please press Forgot Password on the sign in screen.
This will send you an e-mail with a link to reset and choose a new password.