Budgeting your project can be a challenging task if you have not done this before. There are many approaches to establish a good budget plan for your project, the most mechanical one is the Top Down Estimation where you proportionately assign the budget to activities according to chosen attributes. Spending plan is basically an off-shoot of budget plan. After establishing budget & spending plan, budget must be tracked & analysed for changes.
Top Down estimation is good only if you do not have sufficient data for activities otherwise a fair estimation must always be done for a realistic budget.