We didn't always have mod managers. In the old days we followed these simple steps, which still work:

First Time Setup Procedures:

• Find your Steam folder. Go to SteamApps/Common/Skyrim, and create a shortcut of the data folder. Put it on your desktop.
  
• Download and install the trial version of WINRAR. The trial never ends like it's supposed to, so nobody ever actually buys it.

General Installation Procedures

• Download the file manually from SkyrimNexus, PlanetElderScrolls, or TESAlliance.
  
• Right click on the archive, and hit 'extract to _____'. It should create its own folder.
  
• This step varies, depending on how the modder chose to package their file:
  
  • If there is an esp file by itself, and possibly a BSA or readme file:
    
    • Copy these directly to the data folder directory.

• If there is a complex folder structure, and installation instructions:
  
  • Follow the installation instructions the modder gave you.
• If there is a complex folder structure, and zero documentation:
  
  • Attempt a direct copy-paste. Check to see if this works.
• If the above steps have not worked for you:
  
  • Check to see if the download page has installation instructions.
    
  • If there are no instructions, then ask the modder for instructions.
  That's literally all there is to mod installation - I've been doing it this way since Morrowind, and there have only been two mods I couldn't successfully install - Calair Castle in Oblivion, and one of the Magic overhauls for Skyrim. In both cases I was able to quickly find another mod that did the same thing. There are probably a handful of mods that don't support manual installation, but they're pretty rare.