Go to File > Make a copy. Recommend 75% zoom for Sheets.
To use in Excel, simply click the download button then open your file.
To use this spreadsheet, fill in weapons/helmets on the left first, and if you have to buy mounts/adapters/tubes of any kind make sure you list those in the adapter tables below.
Remember not to add mods that your gun comes with if you aren't changing them out. If you DO change them out, subtract the vendor price of the replaced piece for absolute accuracy. Generally only relevant when changing barrels, though.
I was spit-balling my loadout prices before, but honestly was consistently overestimating how important it was to run a cheap gun; most of the cost of a loadout comes from other places like ammo and armor.
I included a basic suppressed vector loadout as an example. Duplicate the original page if you want to calculate more than one loadout at a time.
Edit* Fixed Link