Start with heavy cream at around room temperature rather than straight from the fridge, since cold cream takes far longer to churn and cream that's too warm won't whip into stiff peaks before separating properly. Keep shaking or churning past the whipped cream stage rather than stopping once it looks fluffy, since butter only forms after the cream breaks and separates into solid fat and liquid buttermilk, which takes noticeably longer than making whipped cream. Drain off the buttermilk as soon as separation happens rather than leaving the butter sitting in it, since buttermilk left in contact with the butter sours it quickly and shortens how long the butter stays fresh. Rinse the butter solids under cold water while kneading them, repeating until the water runs clear, since any buttermilk left inside the butter causes it to spoil within days instead of weeks. Squeeze out as much remaining liquid as possible by pressing the butter with a spoon or your hands rather than leaving it moist, since trapped water encourages bacterial growth and makes the butter spatter when you cook with it later. Add salt only after the butter is fully formed and rinsed, not into the cream beforehand, since salting at the end lets you control the exact amount and ensures it's distributed evenly instead of clumping unevenly through the batch.