1. First, download KindleGen from Amazon, and ensure that it’s located somewhere in your path
2. Open a PowerShell window, navigate to the folder that contains your .EPUB files, and run the following command:

Get-ChildItem *.epub | ForEach-Object {&kindlegen.exe $_.Name}

Or, using aliases to save some typing:

gci *.epub | foreach {&kindlegen.exe $_.Name}