Sharing a Laptops WiFi through Ethernet

Here are 3 different ways you can share your laptops WiFi through ethernet to another computer so that that computer can access the internet.
(When your set-up is like the following diagram.)

Desktop --- Ethernet --- Laptop --- WiFi --- Router

A desktop computer connected from its ethernet port to the laptops ethernet port using a cable and that laptop connected to a wireless router via WiFi.
In order of easiest to complicated.