Step 1: Get a kitchen or postal scale. Yes, you need to do this!
You don’t have to buy one, use what you have. If you don’t own a scale, borrow one, or buy one cheap at a local thrift store or secondhand store if possible. If you want to buy one online, consider the AMIR Digital Kitchen Scale, it’s readily available, inexpensive, accurate, easy to use and light!
Step 2: Test it!
Test your scale with objects of known weight. For example, coins (U.S. nickels weigh 5 grams, quarters 5.67 grams), a full SmartWater bottle, or look up the weight of your phone.
What kind of scale do you have? What's the last thing you weighed? What's the next thing you want to do?
Illustration by Mike Clelland from Ultralight Backpackin' Tips by Mike Clelland
I just use an old kitchen scale we already had. For larger items that don't fit neatly on the scale I throw them in a bowl or cardboard box, tare, then weigh.
Just to keep giving kitchen scale tips... if your single item is too small to register on the scale, add multiples until it registers and divide by the quantity.
It's an obvious tip to me in retrospect but helped me and hopefully someone else.