There are a number of ways to download the source code. Some are described here, an alternative all-inclusive guide is also located at https://wiki.yogstation.net/wiki/Downloading_the_source_code
Option 1: Follow this: https://wiki.yogstation.net/wiki/Setting_up_git
Option 2: Download the source code as a zip by clicking the ZIP button in the code tab of https://github.com/yogstation13/Yogstation (note: this will use a lot of bandwidth if you wish to update and is a lot of hassle if you want to make any changes at all, so it's not recommended.)
The Yogstation codebase recommends compiling using version 514.1642 and may potentially NOT work on newer or older versions.
Recently we have changed the way to compile the codebase.
Find BUILD.bat here in the root folder of yogstation, and double click it to initiate the build. It consists of multiple steps and might take around 1-5 minutes to compile.
After it finishes, you can then setup the server normally by opening yogstation.dmb in DreamDaemon. See further down for instructions
Building yogstation in DreamMaker directly is now deprecated and might produce errors, such as 'tgui.bundle.js': cannot find file.