You need to have at least one account created in order for this system to work. Once you have an account, you will see an "available for funding" amount. This amount will adjust as you fund your envelopes.
For instance, if you have $500 in your checking account and you only fund your grocery envelope with $100, you will have $400 available for funding other envelopes ($500-$100=$400).
If you have questions about how the math works, click the "i" next to "available for funding" on the Envelope Balances screen to see exactly how it's calculated.
If you don’t know the exact amount you should fund into each envelope, make an estimate based on what you think you will need to spend. You’ll likely end up adjusting your envelope’s funding as time goes on, so don’t get hung up trying to get it “perfect” as you get started.