If you're interested in using spreadsheets to quickly add LOTS of achievements to the site, read on.
Thanks GTAComplete for the above video tutorial as well.
OK, now the nuts and bolts of how this works. First, take a look at a quick sample spreadsheet I put together for Halo: Reach Achievements
There are 5 columns: Achievement Name, Achievement Description, Achievement Value, Game Name, and Achievement Icon. All these fields should be fairly self explanatory.
Achievement Icons are OPTIONAL though. The weird filename for that column are the filenames you get when saving achievement icons from Xbox.com. This column represents the filename of the image.
In Google Docs, you can simply Share the spreadsheet with me. Click Share in the top right corner, then click Change on the "Private - Only the people listed below can access" line, then click "Anyone with the link" and Save.
At the top of the Sharing settings window will now be a link that you can post on the forums.
In OpenOffice Calc: File -> Save As, then change Save as Type to Text CSV (it's 5th from the bottom). Now upload this file to http://www.hotfile.com/ or any other free file hosting website and post a link so I can download the CSV.
If you added Achievement Icon data to the spreadsheets, you have to send me those files as well.
.zip or .rar all those files and upload them to http://www.hotfile.com/ and post a link to that as well.
Ask as many questions as possible. I'm sure these instructions aren't 100% clear, so I'll be updating this page frequently.