A more recent version of scripts may be available at scripts.irssi.org.
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 sF12When 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.