A morning weigh-in means weighing yourself every day at the same time and under the same conditions, ideally right after getting up. It does not remove natural fluctuations, but it keeps every factor you can control the same.
How to weigh yourself
Morning is when your state is most comparable: after using the toilet, before eating or drinking and before training. Always use the same scale on the same firm floor and wear as little clothing as possible, the same every day. Weighing right after a workout skews the reading because it includes sweat loss that returns with your next drink.
Daily instead of weekly
For a trend weight, daily readings beat weekly ones because more values make the smoothing more stable. If you only weigh once a week, you may happen to catch a day with lots of water and mistake a good week for a bad one. In a study by Steinberg et al. 2015, daily weighing was also associated with more weight loss. A trend still works with only two or three readings a week, but it takes longer to become reliable.
Log every reading, including the high one after a restaurant dinner. A short note about the day helps you place a spike later. If daily weighing feels stressful, look only at the trend and ignore the daily number. After a few weeks you get used to the scatter and stop judging each individual reading.