Tutorial #001 - Making Custom Textures and Packing Them

1.1 Converting to VTF

Hammer has its own filetype to store textures for multiple reasons including compression therefore you cannot simply put your PNGs and GIFs and pray it works!
You need to convert your textures into a VTF file.

To achieve this you require a software called VTFEdit!

Once you've installed VTFEdit, open it and click on File > Import (Ctrl+I)

Select your texture file...

And then apply these settings (for decals you must know the size you want, it will take the maximum size!)

Click on Save As..., name your file and save it in a convenient spot.

Once saved, click on Tools > Create VMT File, then click Create!

2.1 Importing Textures

I would recommend putting your textures in the custom folder of your Source SDK directory.:

Steam > steamapps > common > SDK 2013 > hl2 > custom > CUSTOM_FOLDER > materials

2.2 Finding Textures

To easily find your textures in Hammer(++) i suggest making a sub-folder inside your materials folder
for example CUSTOM_FOLDER > materials > subfolder.

You can easily filter them using the integrated Hammer filter search like so:

Pack your textures using HammerAddons postcompiler

3.1 Installing HammerAddons

Download the latest release here (usually hammer_addons_vx.x.x.zip)

Once downloaded extract the postcompiler folder to:
Source SDK Base 2013 Multiplayer\bin\hammerplusplus\bin

3.2 Setting up postcompiler

In Hammer(++), click on File > Run Map then click on Expert... to pull out the Expert compile options
Add a new step to your compile config.

Move your new step after the vbsp step.

Select your empty step and set the command to Executable

Then navigate to the postcompiler.exe which is in the postcompile folder in either
GarrysMod\bin folder if you use GMod Hammer
Source SDK Base 2013 Multiplayer\bin\hammerplusplus\bin, if you use SDK Hammer++

Now copy and paste --propcombine $path/$file into the command parameters. Make sure the step is ticked on. That's it!

4.1 Conclusion

I hope this tutorial helped you pack your custom textures into your map!
For specific texture making tutorials (animated, etc..) , check this Steam guide.

The postcompiler can also pack sounds and models.
Postcompiler steps and some images taken from the Installation guide in GitHub.