When first opening the app, it will ask you to add a bank account. You will need to enter the current balance, which you can always guess and update later if needed.
Once onboarding is complete, you will see the budget page. Some basic categories are already for you, but you can add more by clicking the arrow on a category group.
Create your first budget by entering amounts into the "budgeted" column of the categories you want to track. Your starting balance will be available as money to budget (see how budgeting works).
That's it! After you've made some more purchases, check back and import your transactions. After categorizing them, you can see how your spending has impacted your budget.
Tip: use the mobile app to categorize your transactions while you're waiting in line for that sandwhich.