This is a complete guide to explaining the process of importing 3D models into your Minecraft world. There are three basic parts I will break the process down into: Setting up you Minecraft, importing/exporting your 3D model, and bringing the model into your Minecraft world.
Step 1: Downloading the Minecraft Addons
Forge and WorldEdit:
Using Forge and WorldEdit is the most basic combination for what we need to do, but it is purely client side (meaning that it will only work in your singleplayer worlds). Even if you plan to import models into multiplayer worlds I strongly recommend downloading Forge and WorldEdit regardless so that you can get a visual scale of your models.
WorldEdit for Servers:
Please note that you need to have a bukkit server to add the WorldEdit Plugin. If you have a bukkit server, just download the plugin and drop it in your plugins folder in the server directory.
Note: MCEdit is also a viable option with a tutorial on how to use it here. Schematica is an additional option, but check your servers rules before using.
Step 2: Installing Forge to Minecraft
Before installing Forge make sure you have run the Minecraft version you are installing to at least once.
1. Navigate to the Forge Installer in your downloads and run it.
2. Make sure the install location is set to your .minecraft folder and "Install Client" is selected.
3. Click "OK", and click it again when the install finishes.
Forge is now installed to your Minecraft.
Step 3: Adding WorldEdit to Minecraft
1. Open File Explorer and type %appdata% into the directory and navigate to the .minecraft folder.
2. In the .minecraft folder, create a new folder called mods.
3. Find the worldedit jar in your downloads and paste it into the mods folder you created.
WorldEdit has been added to your Minecraft.
Step 4: TinkerCAD: a Quick Introduction
Welcome to the next major step to this tutorial! We will now be using TinkerCAD to import our 3D model (or create one) and export it as a schematic for Minecraft. TinkerCAD is a purely web based program by Autodesk, so nothing will need to be downloaded here. All you need to do is create an account and the software is open to use.
Step 5: Importing to TInkerCAD
Go ahead and log into the account you created and select a new project to create. If you have a 3D model, go ahead and select import. If you do not have a model go ahead and make one in TinkerCAD. Remember, the larger the scale, the bigger and more detailed the object will be in Minecraft.
Step 6: Exporting the Minecraft Schematic
Once you have imported the model, Find the Minecraft pickaxe in the top right corner and click it. This will switch you over to Minecraft mode. From here with the three buttons in the top left you can adjust the amount of detail for your model. If increasing the detail to the max does not satisfy what you want, try reimporting the model with a larger scale and try again. With the drop down bar on the right called blocks, you can change the type of block the model will be made of. When you are satisfied, click export and download the schematic.
Step 7: Launch Minecraft Forge
Once your schematic file is downloaded, close out of TinkerCAD and open up Minecraft. Make sure to select the forge version. When Minecraft launches and loads, go back to the .minecraft folder under %appdata%. Click config, then worldedit, and finally schematics. In that folder paste your schematic file you downloaded. Open the Minecraft world you want to paste the model into or create a new world.
Step 8: Adding the Schematic to the Minecraft World
To load your schematic type: //schem load [Name of Schematic]
If you do not find your schematic type the following command to make sure you got the name right, and that it is in your schematic folder: //schem list
Finally, to paste your schematic, type: //paste
Make sure to give Minecraft a minute if you are pasting a particularly large schematic.
Step 9: Congratulations!
There you have it! You modded your Minecraft and imported a 3D model over after converting it.