From the "Home" page of PhotoRoom
Tap on the "Get Pro" button in the top right corner
Choose a monthly or a yearly subscription
Tap on "Continue"
You'll get a free trial period
Validate the purchase
From the Web, you can:
Create an account
Tap on the right upper button
Select "Get pro"
Add your payment details and validate the purchase
If you already have a Pro subscription, you don't have to create a new account for your other devices. Instead, you can share the same PhotoRoom account with multiple devices and get the benefits of going Pro!