Rest your left fingers on A, S, D and F, and your right fingers on J, K, L and semicolon. Both thumbs sit on the space bar. F and J have raised bumps so you can find the position without looking.
Each finger covers a diagonal column of keys. The left index finger takes R, T, F, G, V and B; the right index finger takes Y, U, H, J, N and M. The little fingers handle the outer columns, Shift, Enter and most of the punctuation. After every keystroke, the finger returns to its home key.
Use the opposite hand for Shift wherever you can: right Shift for letters typed by the left hand, and left Shift for letters typed by the right. Using the same hand for both forces an awkward stretch that slows you down and causes mistakes.
Keep your wrists relaxed rather than pressed hard against the desk, sit so your forearms are roughly level with the keyboard, and stop for a moment if your hands feel tense. These are practical comfort suggestions, not medical advice – if you have pain or a persistent problem, speak to a qualified professional.