Note that this app only works for stores that are on Shopify Plus plan.
Once you install an app, go to app countries list configuration where you need to select which countries should be presented with phone verification. Note that only customers from these countries would be presented for phone verification and remaining customers would be able to checkout without any phone verification.
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.