My roommate's cousin left a pack of highlighters on the kitchen table after visiting one weekend, four colors, and I ended up using all four to color-code my spreadsheet before I could tell you what any of the colors were actually supposed to mean anymore.
Your first day
Pull up your last two weeks of transactions, which can take an hour or more the first time. Copy them from your bank app or a downloaded statement onto paper, into a spreadsheet, or just mark them with highlighters. Go through every single line, no skipping small ones, and mark each as needs (rent, groceries), wants (coffee out), savings, debt (loan payments), or subscriptions (anything that charges you automatically). Then open last month's statement and check only for recurring charges, since monthly subscriptions may not have hit in the past two weeks. Add up each bucket at the end and write the five totals down. Don't judge any individual purchase while doing this, the point is the total, not any one transaction.
I nearly skipped the small recurring charges under ten dollars the first time through, since each one felt too minor on its own to bother with. Don't make that mistake. In my case those were almost all subscriptions, and once I added them up across a year they turned out to be one of my five largest categories.
The thing most people miss
I assumed my biggest spending leak was something large and occasional, like eating out too often on nights I didn't feel like cooking. What actually changed my understanding was adding up the buckets side by side and finding my subscriptions total, made of charges I'd genuinely stopped noticing, came out larger than my entire eating-out total for the month.
Once I saw that written down in front of me, I started reviewing recurring charges specifically every month instead of only glancing at big-ticket purchases, since that's where the quiet money had actually been going the whole time.
A misconception worth correcting
The leak in most budgets isn't a splurge you'd remember making. In my case it was a handful of small, automatic charges that never showed up as a decision at all, which is exactly why they were so easy to miss without going line by line.
Categorizing a batch of transactions took fifteen minutes the fourth time I did it, down from the ninety the first pass had taken, and I only noticed because I'd checked the clock out of curiosity both times. A sinking fund, money set aside monthly for a predictable but irregular expense, is worth setting up next, so those costs stop arriving as one-time surprises.