I did a search for this and didn't find anything, but I'm pretty tired so I could have missed it, sorry if I did. I'm just sick of leeching and would like to give it a shot helping others.
Alright, got this from a friend of mine that used this to get into PUGs and the such.
To start you need to find your character's GUID. This is a unique code to your character only. To find this, use this script command.
Type or copy/paste that into your chat window and hit enter. It will output a string that will look somewhat like this./script ChatFrame1:AddMessage(UnitGUID("PLAYER"));
Remove the 0x from the beginning and we are good to go.0x0000000000000000
Now we can start building the achievement itself. We will be using this script here.
The bold spots are the places we will be editing for the achievement./script ChatFrame1:AddMessage("\124cffffff00\124Hachievement:AchNumber:GUID:1:MM:DD:YY:4294967295:4294967295:4294967295:4294967295\124h[NAME]\124h\124r")
Let's choose an achievement. Let's choose the achievement for LK 25 heroic.
The achievement number is bolded in the above quote. The achievement name is [The Light of Dawn].
So now we can edit in the achievement link that information.
Now let's choose a date. Since we are obviously l33t, we will choose the day after Cata dropped, to make it appear we went back at 85 like the l33t ****s we are./script ChatFrame1:AddMessage("\124cffffff00\124Hachievement:4584:GUID:1:MM:DD:YY:4294967295:4294967295:4294967295:4294967295\124h[The Light of Dawn]\124h\124r")
MM = 12
DD = 08
YY = 10
Our achievement is almost done. Now all that is left to do is input your GUID into the script where it says "GUID" and copy/paste the full script into WoW!/script ChatFrame1:AddMessage("\124cffffff00\124Hachievement:4584:GUID:1:12:08:10:4294967295:4294967295:4294967295:4294967295\124h[The Light of Dawn]\124h\124r")
Happy faking!