Once app is installed, only thing you need to do is enable an app block in your checkout editor.

Step 1 : Navigate to Settings > Checkout > Checkout Editor

Step 2 : Click on add app block

Step 3 : Choose "Customer phone verification" block

Step 4 : Optionally you can change various text placeholders here and move around block at other location of your choice.

Step 5 : Save and visit your checkout to test out experience.