I agree with Unicorn67, make sure you weigh the same time every day in comparable clothes (or none at all, just be consistent). I always weigh myself first thing in the morning after using the facilities and wearing the same nightgown.
If I have a glass of water or eat part of my breakfast and then remember that I meant to weigh, I just wait til the next day. I know my weight fluctuates a bit every day (hopefully down!) but I don't want to see it once it's going up from literally just eating or drinking something (of course it would be higher!).
Hope this helps.