Step 1: Set Vernier and arc measure to 0.

Step 2: Point sextant at horizon.

Step 3: Adjust micrometer drum until horizon is absolutely flat. Index error is the result of misaligned mirrors, and can be seen as a split along the horizon; you will see one layer of horizon above another. It may help to move up and down until a comfortable medium is reached.

Step 4: Once a flat horizon is seen, read the vernier. If the reading is less than 0, the correction is said to be "Off the Arc," and therefore the correction should be added, or put on, when completing the sight reduction. If the reading is more than 0, the correction is said to be "On the Arc," and therefore the correction should be subtracted, or taken off, when completing the sight reduction. An easy way to remember this is the saying "If it's off, put it on; if it's on, take it off."