4. Connect
Congratulations on making it this far. All that's left is to fire this sucker up and see if everything works, and try connecting/disconnecting a few times to learn how to do it when you really need it in the future.
The moment of truth
Turn on your iPhone and ensure it is still connected to your computer-to-computer network. Open MobileTerminal.app (available in Installer.app). Simply type slirp and press return. The iPhone should fire up SLiRP. Set it down, don't let it shut off (may have been a good idea to disable this in settings), and leave SLiRP running.
On your mac open terminal and run the following:
sudo /tether.sh
If the terminal gives no feedback after running this command within 15 seconds you should be online, fully able to use just about anything you please. If it works, pat yourself on the back and thank your lucky stars you didn't miss something. If it doesnt...bummer. Read back through everything clearly. PPP logs to /tmp/ppp.log, so check there for issues if you can't get online. If you followed everything here closely and have a basic understanding of the concepts at play, you should be online.
When you wish to close the connection, run the following on your mac:
sudo killall pppd
And on your iPhone, simply type five zeros (0), as SLiRP likely says already on screen. MobileTerminal should drop back to a command prompt, and you're all out.
If you have any issues please carefully review what you've done. Be sure you have an understanding of each step and aren't just shooting blind. If you believe something in this guide is incorrect or can be made clearer, please write webmaster@nsgn.net...but try and do so only if there is something you really think needs change. I can't answer 5000 questions from people who missed a step.
Finally, please visit the credits section. This tethering method is again one I did not create, but rather just took the time to make "easy"(heh). Enjoy your data :)