'Alf's blog' is a zip package of many plugin's.
You need to un-zip the Outer package ONLY to a temporary folder
(DO NOT place these into Calibre's Plugins folder).
(The 'Extract all' instruction assumed
that you had a 'vanilla' windows installation. (there will be some sort of Extract all, somewhere in Your Zip program).
Follow the instructions on choices
From this point, a normal 'Add Plugins' is how you proceed