Import the gpg key used to sign the repository:
Important: the Raspbian repositories are NOT signed.
$ sudo apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 11E9DE8848F2B65222AA75B8D1820DB22A11534E
You must use apt, apt-get or aptitude to install the packages (with sudo or as "root" user).
The following examples use sudo and apt-get.
Install https transport for apt:
$ sudo apt-get install apt-transport-https
Note: the second command line is optional, it is used to download source packages.
Resynchronize your package index files:
$ sudo apt-get update
Install the binary packages; the "plugins" package is highly recommended but not mandatory, scripting API packages are optional (popular languages are python and perl).
$ sudo apt-get install weechat-curses weechat-plugins weechat-python weechat-perl
$ sudo apt-get install weechat-devel-curses weechat-devel-plugins weechat-devel-python weechat-devel-perl
If needed, you can download the source package in order to rebuild binary packages:
$ apt-get source weechat-devel