You create a budget after a deal is won.
A deal is a term we use for a work that can turn into a project but it's still in the sales phase. While negotiating with a client, you have to say what services you offer and for what price and you put it down in a deal.
Checkout Winning the deal article for more information.
You can also add a new budget directly to a project while you're on a Project page. Go to Budgets tab and click the + button.
Give a name to your new budget and hit the Create budget button.
If you're creating a budget manually (not from a deal that was won), first thing you'll need to do is to add Services.
Services define what type of work you do for a client and people working on a project track time against them.
For more information please read about creating services in the Sales section.
If you decide to give your client a discount, open a budget and hover over the + button.
Select Add discount option and put down a number you want to add.
If you change your mind and want to edit or delete a discount, click on the action menu beside a discount and choose one option.
You might have some expenses related to a project. It could be anything from paying contractors to buying licenses, paying for hosting services or travel expenses.
Hover over the + button and select Add expense option.
Put down a name, select a date when you have to pay it and set the amount. You can add an attachment like a receipt or an invoice to the expense.
Make sure you keep all the financial information in the budget accurate (services, discounts, expenses) otherwise your profit and revenue figures won't show the real balance.