Generally, my first recommendation is to look at driver based planning wherever possible, but it will sometimes make the most sense for your budgetholders to put in financial figures directly.
When that's the case - don't just give them a block of accounts and months to fill in and say off you go. Spend some time thinking about how to make it easier for them to be accurate.
Note I'm not saying just make it easier. It's not especially hard to jam in a bunch of numbers to get finance to leave you alone. You want to make it as low a lift as possible for them to put in good numbers.
A good planning tool will let you provide a dropdown, give them the option to key in manually, but also options to put in a figure and spread it evenly, or spread it weighted by prior year - fantastic for seasonality. And everybody has seasonality, even if it's just that things are a little quieter around summer and Christmas.
Other great options are number of days in the month, or - in less material cases - sure, a percentage change from last year.
And also make sure your tool lets you put in data validation. It's extremely helpful if you can make a huge red error colour appear if they put something non-numeric in. Or if they’re going over their allowed amounts, and haven’t put in a comment explaining why. It's a one time investment in your future sanity.
In fact, wherever possible, don't give them the chance to put the wrong thing in - leverage dropdowns pre-populated with valid options from your business model.
I also strongly suggest you add in vertical analysis. If you've not bumped into this one, it's a classic fancy name for a simple thing; it basically means you compare all lines to a base figure. So in opex, for example, each line might have a column for percentage of overall opex spend for that department, or perhaps revenue, or whatever makes sense. It's a fantastic sense check where absolute numbers might not mean a huge amount to your budgetholders.
