Budgeting is simply putting amounts to all the things you'll need to spend money on. Don't underestimate the time it will take to do the work needed to get accurate figures.
Start work on the budget as soon as you can. Budgets should not be guesswork - get as near as you can to the actual amounts you will have to pay. For example: get quotes for building work, get exact prices for equipment, ask how much rent people pay for similar offices to yours, find out how much people get paid for the kind of posts you want to fund.
N.B. Don't forget to include expenses like training (for staff, volunteers, management committee members), National Insurance and pension contributions for paid staff, inflation in future years, volunteer expenses, VAT on goods, services, repairs, renewals, insurance etc.
There are basic costs that nearly every group has, such as postage, stationery, rent, electricity and telephone bills. There may be other costs related to the work your group does, such as transport or catering.
Don't forget to include in your budget realistic amounts for contingencies and reserves.
Funders will know if your budget is not accurate - your figures will be too rounded, or unrealistic. A properly worked out budget is one of the most important elements of successful fundraising.
Some funders will only make grants for capital costs (office equipment, buildings, etc.), while others are prepared to fund revenue costs (salaries and other costs associated with running the organisation or project) Divide your budget up in this way, in order to help you decide which funder to approach for what.
Budgets aren't simply there to help your group raise funds - they are your guide to whether you are spending your money as planned. You should compare your expenditure to the budget every month or so, to keep on top of how much you're spending on what, and to alert you early on that you may not have enough money later on in the year to do what you planned.