Install ioquake3

Install ioquake3, an open sourcing of the quake 3 arena engine, on the platform of your choice, then play with Josh.

Installation on various platforms

Mac

curl -L https://git.io/ioq3mac | sh. I promise it's not crapware. This will create /Applications/ioquake3/ioquake3-1.36.app
Launch the app. You may be denied because it's not a signed app. If so, go into Security preferences and 'Allow anyway'.

Windows

Download installers: 1, 2
Find yourself a copy of pak0.pk3. It's out there on Google...
Run installer 1 and 2 with 'Install for everyone on this computer'
You should now have a directory C:\Program Files (x86)\ioquake3\baseq3. Place pak0.pk3 there.
Launch ioquake3 from the Start Menu.
When prompted for a cd-key, just press Accept.

Customize the game

Setup > Player. Make sure to choose your name and hit the 'Model' button to choose an avatar. There is no benefit for any particular avatar.
Setup > Network, toggle the speed until you hit LAN/Cable/xDSL. Your ping will be horrible unless you do this.
Setup > Game Options, if you want to change your crosshair (i usually do.)

HD / Wide screen on Windows

Go to this path (Start > Run): %appdata%\Quake3\baseq3 then edit q3config.cfg in an editor. Locate the lines which contain the variable names and replace values accordingly. Here are the keys and values to replace with for 1080p. Change as desired, save and exit and then you should be able to run the game with your new resolution.

seta r_mode "-1"
seta r_customwidth "1920"
seta r_customheight "1080"
seta cg_fov "105"

Connect and play

Go back to main menu > Multiplayer
Hit Specify and type quake3.triplehelix.org. Then Connect
You're in!