A budget projection is a set of monthly target amounts against which you can measure the performance of an Income or Expenditure account. These projections can be made at any level of account, ie Category, Summary or Detail.
You will see on the right hand side of this dialog, a column of 12 monthly amounts and a total for the year.
You will see on the left of the dialog 4 push-button functions that operate on these amounts.
A simple method is to set a total budget value and then use Straight Line to spread that total equally over the 12 months.
If you have last year's data available, you may want to use Last Year Actual to initialise the budget to the expenditure level of last year, and increase or decrease the total or individual month's as appropriate.
You can then use Normalise to spread the new total over the 12 months in the same proportion as last year's figures.
You can of course enter individual month values, and then use Add Total.
The budget figures that you set up will be displayed in the right hand column of the account summary.
The budget figures are also displayed when you select the 
function.