To install the custom achievements, simply replace your Cheevos.df with the one provided here.

AS FOR UNINSTALLING THEM
You will first off need to locate and the cheevos.df file from this mod with the one you had previously. You should still have this file - the installing instructions for this mod ask you back up everything before installing(!)

Then, replace the cheevos.df you have currently with the one from your backup.
This should restore your old cheevos.df.
You will then need to find the %appdata% folder.
You can do so by right-clicking on the address bar in file explorer, and then "edit address" - just type %appdata% to get there. 
You'll want to go into the .madgarden folder, and from there, use a text editor to edit "cheevos.save."

If you see any of the following;
	cheevo: green
	cheevo: arklad
	cheevo: frank
	cheevo: meow
	cheevo: monty
	cheevo: halberd
	cheevo: halberd1
	cheevo: halberd2
	cheevo: snrlad
	cheevo: tonee
	cheevo: halberdiergang
	cheevo: unholyalliance

Or anything resembling them, delete those specific lines.
Your game should now work as normal from the achievements perspective.

IF THIS DOES NOT SOLVE THE ISSUE, DM Professor Sycamore (i.e, me) AND I'LL FIX IT FOR YE, DON'T WORRY.