WARNING: FiSH encryption is not secure. See the security section here.
NOTE: A modified version of FiSHLiM is now bundled with HexChat. If you use HexChat you should generally not install this plugin (and if you do you need to remove the bundled version of FiSHLiM).
Type this in a terminal:
sudo apt-get install build-essential libglib2.0-dev libssl-dev
Download and unpack the source code. Type this into a terminal (change ~/fishlim-master to the placewhere you unpacked the source):
cd ~/fishlim-master
make
sudo make install
Start XChat and set up your keys with /setkey secret