Install EveryQuest just like every other addon, drag the EveryQuest folder into the addons folder and run the game. Once you get in however, you're going to need to click a few buttons to get EveryQuest to populate itself with your previously completed quests. To do this go type "/everyquest" in the chat box and then click on the "Options" button. You can get to the options screen a few different ways, but that's the easiest.
Once in the options window go down to "Server Query" and click the "Auto Populate" check box, just as pictured above. Then click the "Query Server" button. This will have EveryQuest automatically query the server for every completed quest and use the results of that query to fill in its own database. The game will appear to hang for a few moments, but that's normal as the server and your client work together. Don't Alt-F4 or get jittery, relax and let things happen.
Once all your quests have been populated into EveryQuest's database, you'll see some information spit out in your chat box, including the number of quests that were added. My neglected warrior had done 2,226 quests, for instance.
Using the AddOn
Through the command line you can access EveryQuest by typing "/everyquest". But the preferred way, at least for me, is to access it through the quest log window by clicking the "EQ" button, as shown above. You'll then be presented with the EveryQuest window that lets you browse quests by zones. Completed quests are shown in green, incomplete quests are shown in white.
With this information you can easily see what quests you have left to do, and discover more information about the quests by typing the quest name into Wowhead. However, EveryQuest integrates amazingly well with the in-game Wowhead addon LightHeaded, and if you have both installed, clicking on the quest in EveryQuest's list will open up the LightHeaded information in your Quest Log window. From there you see if the quest is part of a chain, find what you have to do in order to get the quest, and read the various Wowhead comments about the quest.
There are a bunch of great additional options that you can use to configure EveryQuest's behavior to meet your own exacting needs. If you're using EveryQuest to help you level, you can setup filters to only show quests that are level appropriate. If you're using it to help you complete the questing achievements, like Loremaster, you can have it only display uncompleted quests.
It's worth it to take a few minutes and poke around at the various selections and filters in the options window, especially if you'll be using EveryQuest often.
Kandarz, the EveryQuest author recommends that you install a few other addons with EveryQuest, and I fully agree with him. The recommended complimentary addons are:
You can grab EveryQuest from the usual sources: