It is very simple. Once you download from iOS AppStore (or Android PlayStore) and install the 2ndLine app, please follow the below steps:
Open the 2ndLine app.
Tap Next or Skip intro-screens and tap Continue on the 3rd (last) intro-screen.
Now the app will offer you to Choose your desired number through a screen.
Enter an area code and select a number of your choice OR select from a list of numbers.
Confirm the number
Select a plan of your choice to proceed. You can learn about Subscription Plans through the article.
A successful subscription will make the selected number available for you.
After a successful subscription, the 2ndLine app will request you to grant permissions like Notification, Phonebook, Camera access to make your call, and SMS experience smoother (You can skip the permissions if you don't want to allow). In case you have already granted the permissions, the app will directly show you the home screen.
Once you grant the permissions, you can start making and receiving calls and texts immediately, through your 2ndLine number to a real number.