It's just plain sad how stupid I am, but I can not figure out how to install this mod on my oblivion. The mod is called "Decorator Assistant". I've looked at mod-installation tutorials but I always get stuck somehow. I need someone to hand-hold me through this, step by step, and in the easiest-to-understand terms as physically possible.

Here's what I've done so far:

Selected "download manually"

That downloaded a file that I assume is the mod; it's called "Decorator Assistant-6021.zip"

Most of the guides I've looked at tell me to copy this file into something called "Oblivion Data Files", but I can't seem to find that. Thank you guys for your patience, please help if you can...

I don't have any experience with the precise mod your talking about, but first you need to open the .zip file, as the mod files are inside that. Then you need to find your Oblivion installation directory on your hard drive, it will have a file called oblivion.exe in it and a folder called 'Data'. Your mod files need to be dropped into the data folder. Then you will need to activate the plugin, which you can do by clicking 'data files' on the oblivion launcher, and ticking the box next to your mod. Hope that helps

How do I open the .zip file? Does that mean I just have to click on the folder that was downloaded?

The file you downloaded is an archive with compressed files inside. You need an application like http://www.7-zip.org/or http://www.rarlab.com/ to open the archive and extract the files to your Oblivion\Data folder. I would recommend 7-Zip - it's free to use while Winrar wants you to buy a license after a trial period.

If you downloaded the game from Steam and accepted the defaults, the full path is C:\Program Files\Steam\steamapps\common\Obilivion\Data

I downloaded 7-zip. I opened it, and I clicked on the file I downloaded, and clicked extract. Now I can't find any folder called "Oblivion/Data" or anything relatively close to that. I didn't download from Steam, I have a disc of the game

For the disc copy, the default installation path is C:\Program Files\Bethesda Softworks\Oblivion\Data. Use the 7-Zip File Manager to navigate to it.

Oh my goodness gracious, I finally found it. So I found the Data folder, and I clicked on it and then clicked Extract. Now I can open the Data folder to find the file DecoratorAssistantv1.1, along with some other files called Music, Shaders, Textures, and Video. Is that the right place to put it? So the next step is to "activate the plugin", how do I do that?

Update, I got it (: I found the check box I was supposed to find and marked it, and now the mod works! Thank you so much to both NoOne and Gator, for putting up with my extreme stupidity!

As long as the Music, Shaders, Textures and Video items have folder icons next to them, you should be good. Those are other folders that were installed to the Data folder that the mod needs - the files are inside those folders.

The easiest way to activate the file, if this is the only mod you have, is to launch the game, select Data Files from the launcher menu, and then look for your mod in the file list that comes up. Check the checkbox next to the mod, exit the launcher and then relaunch the game. The mod should be enabled. But, you are really going to need to start using a mod manager if you're going to be adding more mods later. http://www.nexusmods.com/oblivion/mods/2097/?

ETA - I see you figured it out. Still, start getting familiar with using OBMM (the file I linked above). it is superior in every way to the Data Files section of the default game launcher, and is a must-have when your load list grows and you'll need to start managing the order in which your mod files load.