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!
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.
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