HR budgeting is a full process. Usually there is a format to put in the proposed expenses during the year.
You need to check what all come under preview of HR. Eg. Recruitment, Training, Salaries, Statutory compliance. Sometime there are some Admin functions that are also part of HR. Also, take expense figures from finance for the previous year to understand the trend.
The expenses are than broken in to monthly/ quarterly figure. Example, if you propose to have 4 training programs during the year, which will cost 4 lacs. Than you plan when you would have these programs and put the expenses in that month/ quarter.
Based on your proposal, Management decides how much money they can spare for which activity and which month. Finance tracks these expenses and you get monthly budgeted/ actual expenses.