You can use InvoiceOwl on the web or download the mobile app for your phone.
Web app (no download needed):
Go to app.invoiceowl.com in any web browser to get started.
iOS (iPhone and iPad):
Download from the Apple App Store: https://itunes.apple.com/app/id1521414172
Android:
Download from the Google Play Store: https://play.google.com/store/apps/details?id=invoice.maker.estimate.billing
Search for "InvoiceOwl" in either store to find the app. Your data syncs across all platforms, so you can use the web app and the mobile app with the same account.
On the Mobile App
After downloading, open the app and you will see the Welcome screen with a carousel of value props. Tap Get Started to create a new account, or tap Already have an account? Log in to sign in with your existing credentials. The mobile app supports sign-in with Email, Google, and Apple (iOS only).