This is not as complicated as that video makes it out to be.
- Flash OpenWrt to a SD card, install card in the Pi and boot it up.
- Connect your PC to the built-in Ethernet port.
- Create a wan network named exactly 'wan'-- this is important because that name already exists in the firewall. The protocol is DHCP client.
- If you used a community build with the USB Ethernet adapter driver already included, you will see eth1 in the interfaces, proceed to step 4.
- If you used an official build you will need to temporarily connect the Pi to the Internet to install the driver. The simplest way to do this is to start up the Pi built in wifi in client mode and connect to your home wifi network. Attach the wifi client to the wan network and install necessary packages. Confirm that there is an eth1 device in the list. You may need to re-plug the adapter and/or reboot.
- Change the wan physical device to eth1. Plug an Ethernet cable from the USB adapter directly to your modem or other source of Internet. The Pi should now be routing from your PC to the Internet.
- If you want wifi access, it's usually possible to re-purpose your old router to be a "dumb AP" and unmanaged Ethernet switch, even with stock firmware. This will be better wifi performance than the Pi built-in wifi. Plug the Pi built in port to the AP LAN port. Plug any wired PCs into the other LAN ports.