Most people probably know this but thought I would post it for the one's who don't, Also searched the forum and found no threads on this apart from the old way back in 08/09
Step1: Wowhead the achievement as an example I'm using Glory of the orgrimmar raider,
Glory of the Orgrimmar Raider - Achievement - World of Warcraft
Step2: Click links and Copy the wow head url
Step3: Go to this website: WoW Achievement Generator, Fake WoW Achievement
Step4: Paste Wowhead url into the empty box with the dates next to it,
Step5: Adjust the dates don't have it the same week. (also suggested putting it before the 19th if its SoO garrosh kill or they say its looking for raid)
Step6:Click Generate, You should get something along the lines of
/run DEFAULT_CHAT_FRAME:AddMessage("\124cffffff00\124Hachievement:8454:"..strsub(Unit GUID("player"),3)..":1:10:10:13:4294967295:4294967295:4294967295:4294967295\124h[Glory of the Orgrimmar Raider]\124h\124r")
Step7: Copy and paste this writing into you chat box on wow ensuring the /run has nothing behind it , An achievement will show once you hit enter. (nobody else will see this)
Step 8: Shift Click this achievement to link it in the chat,
Any questions Leave a comment!