In the server computer:
1. On the computer connected to the Internet, click the network icon in the panel and go to "Edit Connections..." at the bottom of the menu.
2. Double click your Wired Connection (Leave your wireless connection untouched, the one connected to Internet and the one you want to share, as I understand)
3. In the "IPv4 Settings tab", select Method: "Shared to other computers"
4. Reconnect clicking on the Wired Network so it gets its new IP address. (The two computers must be connected by an ethernet cable for this step, so connect them now if you have not already.)
5. Click on "Connection Information" in the network menu and write down the IP address and network mask (in my case it was assigned 10.42.0.1/255.255.255.0 but I do not know if that will always be the case)
In the client computer:
6. Go to "Edit Connections..." and assign a "Manual" Method. Assign an IP assress in the same subnetwork (10.42.0.69 for example) and put the IP and network mask you wrote down in "Netmask" "Gateway" and "DNS servers"
7. Reconnect to the network to let the new settings be assigned.
8. Pat yourself in the back and surf away!!!