I'll use ILSpy as an example, I personally use that.
Go to their GitHub page ( https://github.com/icsharpcode/ILSpy ) and scroll down to the readme section.
Click on "latest release", and at the "releases" section, scroll down until there is the green "latest release" tag. You see the "...binaries..." zip file. Download it.
Then, unzip the zip file. You have ILSpy.exe as the decompiler executable. No explicit installation required, just double-click on that exe and the decompiler will run itself.