I want to create some custom signage, like the white signage Valve created. Only problem is, I have no idea how. Is there a tutorial somewhere?

The lightboards you see at the start of each chamber is a VGUI and can be changed by editing sp_lightboard_icons.txt within portal2/scripts/vscripts and putting your own mapname with the icons you would like.
The main problem with doing it this way is the file cannot be packed into the .bsp of the map and therefore you will not be able to see it on the workshop.

Omnicoder created a chamber sign maker that will produce a custom texture for you to use. See here.

Follow this tutorial for the custom texture way: mapping-help/custom-chamber-signs-without-scripts-t3663.html
and this one https://developer.valvesoftware.com/wiki/Test_Sign if you want to use sp_lightboards. Please note that VPK's no longer work.

Basically, in order to make a custom texture appear into the game you need 2 files:

1.) The "Valve Texture Format" (.VTF). This is your texture or drawing itself.

2.) The "Valve Material Type" (.VMT). This is the configuration of your material or texture. Without this file and the correct path inside to your actual texture, the game won't be able to find your custom material and you won't be able to find it in your texture browser

Any help you need on this do not hesitate to ask me

And, of course, you'll need some image creating software, though I'll go out on a limb and say you've already got that. If not, some good ones would Photoshop, GIMP, even MS Paint could do the trick if used effectively. Then of course you'd need to export that to .VTF, you'd use VTFEdit for that. VTFEdit also can create a .VMT for you. Then you plonk both in your materials directory, and you're good to go. Don't forget to PakRat, etc.

srs bsnss wrote:
... even MS Paint could do the trick if used effectively.

Really? I don't like it at all I thought you would at least mention Paint.Dot.Net I love it, it's the one I use.

I've been using Paint.Net