You can download SQL Server from this page: https://www.microsoft.com/en-us/sql-server/sql-server-downloads There are a couple of offerings. Since I don't your needs, I can't suggest which offering to choose.
Once you have selected an offering you click link to get a download stub. When you run it, you will have three options: Basic, Custom and Download Media. I recommend that you select the last option.
Click the ISO file to mount it and then run Setup. When you come to the screen to select features, only select Database Engine.
If you have any problems beyond this, you need to be more specific.