Create one
Budgets → New budget. Give it a name and a year, then a planned amount per account per month.
A budget starts as a draft. Activate it when it is ready — only an active budget is compared against. Archive last year's rather than deleting it; comparing years is most of the value.
Seeing the comparison
Reports → Budget vs Actuals puts planned next to actual per account, with the difference. Every actual comes from the ledger, so it moves as transactions are categorised — nothing to update by hand.
What it is for
Not judgement after the fact. A budget you look at in March is how you find out you are 40% through the year and 70% through the marketing line, while there is still time to do something about it.