Each mod can be a little different in how to install them.
The bottom line is this :
– create a folder on your machine (on your desktop for instance). Call that folder ‘Install’.
– once you download a new mod, extract the files to that Install folder.
Now look at the Install folder.
.esp / .esm files need to go directly into Oblivion/Data folder.
Any folder such as : meshes / textures / sounds need to go into Oblivion/Data as well.
Basically, if you see a folder that already exists in Oblivion/Data, you will have to copy the content to that folder.
Now, depending on the way files are archived for a particular mod, you will find some of the following situation once you extract your files into Install:
Install / / Data
or Install / Data
or Install
You will have to explore the Install folder and find the right files and folder that you will have to copy into Oblivion / Data.