Pick a simple method: notes app, WhatsApp-to-self, a tiny notebook, or any basic expense app. Create 5–6 broad buckets (Food, Transport, Home, Health, Personal, Misc).
Log it right away (takes ~10–20 sec): amount + bucket + one word (e.g., “coffee,” “auto”). For cash, snap a photo of the bill if you’re in a rush and write it later.
Weekly 10–15 min check: total each bucket, circle 1–2 leaks and set one tiny tweak for next week (e.g., “auto twice → metro once”).
For today only, note every spend in one list. Add buckets this weekend.
Never store OTP/PIN/card numbers. Be cautious with apps that read SMS or need wide permissions. If money tracking spikes anxiety, track totals by bucket rather than every line, and take a breather day.