1. Copy/symlink script as ~/.weechat/python/fullwidth.py
2. Run /script load fullwidth.py in weechat
3. Amuse yourself with your new ability to write ＬＩＫＥ　ＴＨＩＳ.

Optional:

Symlink into ~/.weechat/python/autoload to load automatically.

Simply use the command like so:

/fw THE CANDY HORSE NEEDS SNUGGLES~

to produce the output:

ＴＨＥ　ＣＡＮＤＹ　ＨＯＲＳＥ　ＮＥＥＤＳ　ＳＮＵＧＧＬＥＳ～

There's really nothing to it.