1. What you need

Windows 10 or 11 (64-bit), Arma 3 on Steam, and the portable zip nimby-0.9.0-win-x64.zip from the releases page.

There is no installer, no registry entry and no administrator prompt. The .NET runtime is bundled, so you do not install .NET either. Nimby writes only to the folders you point it at, its own configuration folder and the scratch folder.

The build is not code-signed, so SmartScreen may warn on first launch. Check the download instead of trusting it; SHA256SUMS.txt is published next to the zip:

$zip = "nimby-0.9.0-win-x64.zip"
$expected = ((Get-Content .\SHA256SUMS.txt) -match [regex]::Escape($zip))[0].Split()[0]
(Get-FileHash .\$zip -Algorithm SHA256).Hash.ToLowerInvariant() -eq $expected.ToLowerInvariant()

The last line prints True when the file is intact. Only then extract it, and only then click past SmartScreen.