Windows
Download the archive with editor and plugins then just unzip the archive to any destination.
Ubuntu
For Ubuntu 10.04 "Lucid Lynx", 10.10 "Maverik Meercat", 11.04 "Natty Narwhal" and 11.10 "Oneiric Ocelot" do the following steps in terminal:
Stable version:
$ sudo add-apt-repository ppa:mezomish/juffed
Development version:
$ sudo add-apt-repository ppa:mezomish/juffed-dev
Then
$ sudo apt-get update
$ sudo apt-get install juffed juffed-plugins
Fedora
$ cd /etc/yum.repos.d
Then
$ sudo wget -nd "http://download.opensuse.org/repositories/X11:/QtDesktop/Fedora_15/X11:QtDesktop.repo"
or
$ sudo wget -nd "http://download.opensuse.org/repositories/X11:/QtDesktop/Fedora_16/X11:QtDesktop.repo"
Then
$ sudo yum install juffed juffed-plugins
OpenSUSE
Gentoo
Arch