I do a zero-based budget monthly, where I precisely account for every dollar coming in and going out. Some of the categories in my budget (anything I buy in person) are designated as "cash" - every paycheck I withdraw enough from the ATM to cover the next two weeks worth of cash categories. These are then distributed into envelopes per category.
All of this leads up to my answer: I carry around personal allowance (budgeted for!) in my wallet at all times, and extra cash pulled from the envelopes when I'm going to buy something that day.
(When I end up using my card for a cash expense, because I didn't anticipate needing to buy something that day, the cash goes into a separate "Return To Bank" envelope. The next time I would withdraw cash, I just take what's in that envelope first, and then withdraw only what I still need. In other words, using my card is merely a loan taken out against my next cash withdrawal.)
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)