Click "Settings", then go to the "General" section. Scroll down a bit, then choose "VPN". Click "Add VPN Configuration". Now set the type to "PPTP" -- this will change the options available. Configure the following:
Description: This is just a name, we suggest you use "YF PPTP" or whatever you like.
Server: This is the name or the IP address of one of our servers. See the general page for details.
Account: Your username.
Password: We suggest that you configure it here, but you can also type it in manually every time you make a connection.
This is all you have to do, for everything else the default settings are fine. Click "Done" when done to save the configuration. iOS will warn you that using PPTP VPNs may not be safe. This is technically correct, see the general page for notes about this. If, however, you use a strong password, it is unlikely that someone will be able to read your traffic -- we therefore advise that you do not use a password that might appear in a dictionary list and just add some characters to it -- use something really cryptic.
To start the VPN connection, go back to the "Settings" level (or press the home button and click "Settings"), then click on "VPN", select your freshly configured VPN profile (you can have more than one, for example with different YF servers), and then toggle the switch to get connected.