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
For big items like your loaded pack, you can weigh yourself with and without the pack and subtract the lower number from the higher number. The difference is your pack weight.
Yup, great advice. I've weighed a pack on a luggage scale before but they're not worth buying for ultralight backpacking, as you can track your items in something like lighterpack or a spreadsheet