A very simplified approach because you were asking for a startup. I think the following should suffice. you should start with a brief financial forecast (preferably monthly) for the next 3 year as follows:
Direct Costs (Raw Material, Direct Labour, Direct Overheads)
Indirect Costs can be further broken down as under:
PF / ESI Contributions
Compliance (Audit, filings etc)
Website / Hosting
d4. Finance Costs
for each of the head above define approximately what you may spend per month. (in case of an existing business, please analyze the last 3 years / 36 months to get an average in case you need). you can also take a zero-based approach and estimate it afresh while it requires detailed analysis.
consider Inflation for future year forecasts.
once you have done, then you are ready with your budget. you can further break down the budget to “Cash Flow Budget”. I will cover this in another article.