Set the overall amount before the category totals
It generally works better to decide on a total amount you're comfortable spending across the whole season first, and then divide that total among categories, rather than adding up categories individually and hoping the sum feels reasonable. Starting with the overall number keeps every other decision anchored to something concrete instead of growing one purchase at a time.
This also gives you an early warning sign. If the category totals you'd like to spend already exceed the overall amount before you've bought anything, that's useful information in November, not a surprise to discover in January when the statements arrive.
Gifts are usually only part of the total
Gifts tend to get the most attention, but a realistic holiday budget usually needs room for food, travel, decorations, shipping costs, cards, and other smaller categories that don't get planned for individually but add up across the season. Listing these categories out, even roughly, before spending begins makes it far less likely that one of them catches you by surprise in December.
Shipping in particular is easy to underestimate, since it's often paid a few dollars at a time across many separate purchases rather than as one visible total. Adding a rough shipping estimate to the plan up front tends to be more accurate than assuming it will stay negligible.
Leave some flexibility in the plan
A budget built with zero room for anything unplanned tends to break the first time an unexpected invitation, a last-minute gift, or a shipping delay requiring an alternate purchase comes up. Building in a small cushion, rather than allocating every dollar to a specific line item in advance, makes the plan more resilient without requiring you to abandon it at the first surprise.
The size of that cushion doesn't need to be large to be useful. Even a modest amount set aside as unallocated can absorb a small surprise without forcing you to pull money from a category you'd already planned to use fully, and without needing to rework the whole plan over one unexpected expense.
A spending plan is different from a wish list
It's easy to build a list of everything you'd like to buy for everyone on your list and call that a budget, but a wish list and a spending plan serve different purposes. A spending plan reflects what you've decided you can actually spend, category by category, while a wish list reflects what would be nice in an unconstrained world. Keeping the two separate makes it easier to see where the wish list needs to be trimmed to fit the plan, rather than the other way around.
Writing both down separately, even briefly, makes the gap between them visible early, while there's still time to make a deliberate choice about which items on the wish list actually fit and which ones can wait for another occasion.
Revisit the plan as actual spending happens
A holiday budget set once in November and never looked at again isn't doing much work. Checking back against actual spending as purchases happen lets you see early whether a category is running ahead of plan, while there's still time to adjust something else rather than discovering the gap only after the season is over.
A short check-in every week or two during the season is usually enough. The goal isn't to track every purchase in detail, but to notice early if one category, like gifts or travel, is running well ahead of what you set aside, so you can adjust a different category before the season ends rather than after.