The Nexus Mod Manager (NMM) is recommended for proper installation. If you use NMM, just click the "Download with Manager" button and install!
My mod comes with a fancy installation script that lets you pick and choose which parts of SMIM you want and pick various texture options. Before requesting options please try the different options through Nexus Mod Manager!
The Official High-Res DLC is OPTIONAL, but only a lunatic wouldn't use it. Just download the High-Res DLC from Steam and activate the .esps for it.
Also download the Unofficial High Resolution Patch.
The Unofficial Skyrim Patch is recommended. Install order doesn't matter since the USKP is a BSA package meaning SMIM's loose files will always take precedence.
The Unofficial patches for Dawnguard, Dragonborn, and Hearthfire are also extremely recommended but in no way required. Do use them though since they fix some very serious bugs with the DLCs.
If you use WATER (recommended!), install it AFTER SMIM and let it overwrite all SMIM files. This ensures you get the animated blacksmith troughs and wetter rocks.
If you use SkyFalls and SkyMills (recommended), install it BEFORE SMIM and then let SMIM overwrite all files. Otherwise, one of the windmill fans will remain 2D.
If you use Better Dynamic Snow (recommended!), install it first! Then let SMIM overwrite all BDS files! Otherwise, you won't get 3D ropes for the farmhouses.
If you use raiserfx's Ruins Clutter Improved (recommended!), install it first! Then let SMIM overwrite all Ruins Clutter Improved files! Otherwise, you won't get SMIM's fancy lantern or various Nordic ruins improvements.
If you use rheadude's Ultimate HD Fire Effects, install it first! Then let SMIM overwrite all files!
If you use kryptopyr's Complete Crafting Overhaul Remade or Tytanis' Jewelcraft, install it first! Then let SMIM overwrite all files!
If you use anamorfus' Enhanced Lights and FX, install SMIM first then pick the SMIM Meshes installer option for Enhanced Lights and FX.
If you use texture mods like Skyrim HD, install SMIM first, then the texture mods and let them overwrite the few files that might conflict with SMIM.
If you use Mindflux's ENB Particle Mesh Fixes, let SMIM overwrite everything because SMIM already includes all his fixes.
Manual Installations and Other Notes
Manual installations are extremely discouraged since SMIM's folder structure is very complex. If you insist against all better judgment, you can download the SMIM archive and then extract the contents of the "00 Core" folder to "SkyrimData". That installs the base part of the mod.
The other folders starting with numbers are optional features. If you want one of those options, extract the contents of the folder to "SkyrimData". Folders with the same number are either or, so only extract the contents of one of those folders. If you can't figure out all the numbering and folder descriptions, use NMM or Wrye Bash. Version 1.11 and above includes an .esp. Be sure to activate it.