"LightmappedGeneric"
{
   "$basetexture" "texture_name_here"
   "$surfaceprop" "glass"
   "$selfillum" 1
   "%keywords" "portal2"
}

A vmt is just a text file (example above). Put this one in portal2/materials with the vtf (give them the same name,probably your map name). Edit the quotes after basetexture and add the texture name + any folders after the materials folder.

Then it'll show up in hammer like every other texture.

The rest of the tutorial explains how to set it up to flicker on like the real ones.

You'll also need to pack the files into your map so players can see the textures.

"LightmappedGeneric"
{
   "$basetexture" "directory/vtf_file_name"
   "$surfaceprop" glass
   "$selfillum" 1
        "%noportal" 1
   "%keywords" "portal2"
}

Replace directory/vtf_file_name with your actual directory. For example, if my .vtf directory is in materials/RectorRocks/Testing for Fun/Test Chamber 01 Sign.vtf, then you should put it as "RectorRocks/Testing for Fun/Test Chamber 01 Sign" Remember to include quotation marks " " and not include ".vtf".
Step 5: Save the notepad file as a .vmt (after the name of the file, put in .vmt)
Step 6: Drag both the .vtf and .vmt in the folder.
Step 7: Open Hammer, go to the Texture Browser, filter by typing your sign name, select and voila.

If you're still having difficulties...
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Here is a tutorial to help you

Cheers!