Just going into this, I’d like to say that this probably isn’t the best way to do it. It’s very inefficient with materials so this technique may be unsuitable for heavy use in game engines like Unity.
Let’s get started. I’m going to assume you know a bit about Blender, but you could probably go in as a complete beginner and understand it.
This tutorial also suggests that you have a folder of textures. A good resource for textures is CGTextures, if you didn’t already know.
We’ll start with building a material library.
Start up Blender.
Select the default cube.
Open the Materials tab.
Name the material whatever you like. Preferably something that’ll make it easy for you to pick out of a crowd.
Go to the Texture tab.
Name your texture like you named your material.
Change the texture type to “Image or Movie”
Find your texture folder and import the texture you want.
Add a new material slot.
Add a new material.
Do what you did over again until you have all of your textures imported.
Save your .blend file. Do NOT delete the cube.
Your material library is now complete. Now onto the actual mapping.
Open a new file.
Click append in the file menu.
Find your material library.
Click on it to navigate through its files. Click on the “Materials” folder.
Select all of your materials. You probably have more than me.
Your materials are now selectable in the materials tab.
Drag out the side menu for a moment.
Change the Material Mode in the Shading sub-menu to “GLSL.”
Close the side menu.
Change into Edit Mode.
Change the Viewport Shading to “Texture.”
Build your map. I’m not going to teach you how to use blender, you can do that on your own time. (You’d probably want to google “Blender tutorial” for this)
You’d want to separate your map into different pieces, like you would in Hammer, but I’m going to keep it simple for the tutorial purposes.
Make sure all the materials you need are in the materials of the Brush/Model. Basically, if you need 2+ materials, you click the + icon and find the material you need in the dropdown menu found underneath the material list.
Select the faces that you want to texture.
Select the material you want and click “Assign.”
Go the the Shading/UVs tab, click the UV Mapping dropdown, and select “Cube Projection.”
Boom, the faces are textured.
Repeat for the rest of the model until all the faces are textures with the material you want them to have.
If you want to mess with the UVs to make them bigger or smaller, google “Blender UV” or something similar. There’s no lack of Blender tutorials online.
That concludes this tutorial. I’m sure there’s a better and faster way to do this, but I’ve never seen a more workflow-efficient way. Tell me if something’s hard to understand and I’ll see if I can clarify.