Spoiler: Downloads

Download Fabric Installer (should be a .exe file, only needed if using default launcher)
Download Fabric API
Download Litematica Fabric and Malilib Fabric (should be 2 separate .jar files)
*** OPTIONAL OPTIFINE ***
Download Optifabric
Download Optifine (make sure to download a 1.18 version - should be a .jar file)

Spoiler: Installing Mod - Default Launcher

*** INSTALLATION ***
Run Fabric Installer (most default settings should be fine - make sure Minecraft Version is "1.18" and "Create profile" is checked) click Install and close the installer.
Open Minecraft Launcher with the newly created Fabric profile.
Once on the main menu, go to Options > Resource Packs... > Open Resource Pack Folder.
Close the game.
In File Explorer go up a folder (should be the .minecraft folder)
In the "mods" folder, place the .jar files inside (should be Fabric API, Litematica Fabric, Malilib Fabric and optional Optifabric and Optifine .jar files)

*** TROUBLESHOOTING ***
If updating the mod, remove all .jar files in the "mods" folder and in the "versions" folder delete the "fabric-loader..." folder. In the Minecraft Launcher, go to the Installations tab and next to the Fabric profile, select the 3 dots and choose delete. Then repeat the above listed steps.

Spoiler: Installing Mod - MultiMC

*** INSTALLATION ***
Login to your account on MultiMC (top right)
Follow Steps 1-2 on this page.
Close the game
In MultiMC on the right, select Minecraft Folder
In the "mods" folder, place the .jar files inside (should be Fabric API, Litematica Fabric, Malilib Fabric and optional Optifabric and Optifine .jar files)

*** TROUBLESHOOTING ***
If updating from an older instance, remove all .jar files in the "mods" folder and in MultiMC's "Edit Instance" remove both the Intermediary Mappings and the Fabric Loader and repeat the above listed steps.

Spoiler: Controlling Litematica (with pictures)

Spoiler: Copying/Saving Schematic

Let's copy this village

Select the "tool item" (by default, uses a stick - can be changed in Generic Configs M+C)
Select different modes by holding left-control+wheel

Open Selection Manager M+S
Create and name a new area

Add a selection box to the selected area M+A
Two boxes will be created on your position


The bottom box (light blue - also currently selected box) is the origin point (reference point when placing the schematic)
The top box is the area selection (we'll cover that soon)
Left-click to move selected box (origin point)

Middle-click to select/deselect boxes (origin point turns orange)

Select area selection Middle-click
Left click to select first point, right click to select second point (sneaking while selecting will place point on highlighted block)



Control+Alt+S to save schematic

Spoiler: Placing the schematic

Now to place the schematic

Manage Schematic Placements M+P
Select "Show loaded schematics"
Select "Load schematics to memory"
Select your schematic in the list and "Load Schematic to memory"
Schematic will be placed on your location based on the origin point


Manage Schematic Placements M+P
Select "Configure" on current schematic
Move selection by coordinates, rotate selection and view material list (can also be locked to prevent accidental movement)
Selection can also be moved by holding left-alt+wheel along the X/Z coordinates based on your view

Spoiler: Controlling Litematica (without pictures)

Spoiler: Copying/Saving Schematic

M+C Config menu - change tool item, modify colors, render layers of the schematic)
Equip tool item (stick by default)
Left-control+wheel to select tool mode
M+S to create a new selection area
M+A to add a selection region to the selected area
Two boxes spawn on player (bottom box is origin point used for reference when placing the schematic - top box is area selection)
Selected box is light blue (origin point turns orange when deselected)
Left-click to move origin point
Middle-click to select/deselect boxes
Left-click to select first point on area selection - right click to select second point on area selection
Control+Alt+S to save schematic

Spoiler: Placing the schematic

M+P Manage Schematic Placements
Select "Show loaded schematics"
Select "Load schematics to memory"
Select your schematic in the list and "Load Schematic to memory"
Schematic will be placed on your location based on the origin point
M+P Manage Schematic Placements
Select "Configure" on current schematic
Move selection by coordinates, rotate selection and view material list (can also be locked to prevent accidental movement)
Selection can also be moved by holding left-alt+wheel along the X/Z coordinates based on your view

More information on the mod can be found here

 

@Cas_with_a_K

 

Updated instance with a newer version of rift/litematica. Also includes Optifine. Thanks @FrailDK

 

Updated download links to 1.14.4 and included optional Optifine downloads.

 

Links updated to 1.15.2

 

Updated thread to include Default Launcher installation and separated MultiMC installation into a separate section.

1.15.2 Optifine preview added.

 

Thread updated to 1.16.3.

 

Updated to 1.17

 

Updated to 1.18

 

Updated to 1.18.1

 

Updated to 1.18.2

 

Press M+C to go to your config menu. Open Hotkeys and scroll down to "pickBlockLast" and set a hotkey. You can combine this with an autoclicker to constantly have the next block be placed in your hand.
Also under the Generic tab on the config menu, you can scroll all the way down to "pickBlockableSlots" and choose where in your hotbar you'd like the blocks to be moved to (so your tools aren't moved around). The hotbar numbers are left to right, 1-9.