Binding F-keys

First of all, read the section of the startup howto about keybinds (again).
Then, if you're too lazy to do all your F-keys manually, just copy-paste this in your irssi:

/bind meta2-P     key F1
/bind meta2-Q     key F2
/bind meta2-R     key F3
/bind meta2-S     key F4
/bind meta2-15~   key F5
/bind meta2-17~   key F6
/bind meta2-18~   key F7
/bind meta2-19~   key F8
/bind meta2-20~   key F9
/bind meta2-21~   key F10
/bind meta2-24~   key F12
/bind meta2-23~   key F11
/bind meta2-24~   key F12

/bind meta2-2P    key sF1
/bind meta2-2Q    key sF2
/bind meta2-2R    key sF3
/bind meta2-2S    key sF4
/bind meta2-15;2~ key sF5
/bind meta2-17;2~ key sF6
/bind meta2-18;2~ key sF7
/bind meta2-19;2~ key sF8
/bind meta2-20;2~ key sF9
/bind meta2-21;2~ key sF10
/bind meta2-24;2~ key sF12
/bind meta2-23;2~ key sF11
/bind meta2-24;2~ key sF12
When you've done that, you can do, for example /bind F1 change_window 1 to bind something to F1, or /bind sF1 for shift-F1.