Set out the ingredients for the dish you've decided to cook.
Add each item to the app, using the the nutritional information on the packaging. If it's something that doesn't have the information on the packaging, as can be the case with fruits, for example, or if you've bought loose fruits or vegetables, you can find the information online, on the supermarket's website.
Then weigh out everything you need to cook that dish. Not herbs and spices, but everything else. Log every ingredient you've weighed for that one dish under the same meal. Doesn't matter whether it's breakfast, lunch, dinner, or whatever.
Then cook.
Once the food has cooled down, weigh the whole thing. Make a note of the number of grams.
Go back to the app, press the 3 dots menu, select "edit diary" and tick every entry you made for that meal. Then press the 3 dots menu again and select "save meal". I find it helpful to include the total weight in the meal name.
Now you know how much the entire dish weighs, you just need to weigh out each portion of it that you serve yourself. When you want to log it in the app for tomorrow's dinner, or whatever, you will just pres "add food", then "my meals", then choose the meal (but don't press the "+" icon). Press where it says "number of servings", and enter the fraction of the dish you've served yourself. So when you weighed the entire cooked dish, say it weighed 1,000g. If you served up a 350g portion, you'll enter 0.35.
Now you have an accurate (kind of...) log of your calories. It's only "kind of" accurate if you are the only person who is going to eat all of the portions.
I hope that helps! I know it's really long and it might sound complicated, or something, but once you get into the habit of doing it, it's fine. I won't lie, though – it is boring and it does take time.
If you are keen to lose weight, I'd recommend logging everything that has more than negligible calories. Some people do not count vegetables, but they do add up over the week.
Anyway, yes, I hope this helps you!