Check the oil level against the marks engraved on the dipstick.
The car should be on level ground when you check the oil, otherwise the oil-level dipstick will show a false reading because the sump into which it is dipped will be at an angle.
The dipstick is usually in a tube on the side of the engine, where it is easy to reach. Before pulling out the dipstick, always clean its handle and the top of the filler tube to stop dirt falling into the tube, and so into the engine.
Withdraw the dipstick and wipe it with a clean, lint-free cloth or tissue paper to remove all traces of oil.
Put the dipstick fully back into the tube and then withdraw it again to check the oil level.
Hold the dipstick horizontally so that the oil does not run off. The level should be at the Full/Max mark. If it is not, add oil to bring it up to the correct level.
Take off the oil filler cap on the engine rocker or cam cover, and pour in amounts of fresh engine oil, of the correct grade, checking the dipstick level frequently to ensure that you do not put in too much.
When checking during refilling, always allow a minute for the oil to drain into the sump, so that the dipstick reading is correct.
When the oil level reaches the Full/Max mark, replace the dipstick and the filler cap.