To start off, the legality of this practice must be established. According to section 6B on the Steam Subscriber Agreement, workshop submissions may be freely downloaded by users, and there are no specified terms that would not allow this. There are many valid reasons to want to download an addon directly. Looking at how to successfully structure an addon that works, decompiling it to learn how it was made, etc. There also many invalid reasons to download an addon, such as reposting it, stealing credit, etc. With great power comes great responsibility.
1. Picking your mod
To download an addon, you need an addon. You must have the URL of the page, which will look something like this. https://steamcommunity.com/sharedfiles/filedetails/?id=1569496117 Once you have that, it's ready to be downloaded.
Downloading addons directly is not supported by Steam, and thus we need a third party website to do this. We will be using steamworkshopdownloader.io for this, there are many websites that can be used but I have had the most success with this one.
Take your workshop link, and paste it into the textbox. It should identify the addon and give you the option to download it.
When you click download, after a few seconds to a few minutes (depending on the addon size) you will get a .zip file with the addon inside.
3. Per-game compression
Some workshop addons are compressed using proprietary means, such as .GCF files, .GMA files, .VPK, etc. There are many that exist, and you will have to do your own research. This can be as easy as just installing one program, though.