In debian testing and sid and in the ubuntu packages are in the official repositories.

sudo apt-get install psi-plus psi-plus-plugins psi-plus-icons psi-plus-skins

Your system may not be libqca-qt5 and Qt>=5.6.0. If you do not - they must be installed prior to build.

Install packages to build Psi +. Installed once at the first build:

$ sudo apt install build-essential libhunspell-dev libminizip-dev libqca2-dev libqca-qt5-2-dev qt5keychain-dev qtbase5-dev qttools5-dev libidn11-dev qtmultimedia5-dev libqt5multimedia5-plugins libqt5x11extras5-dev libqt5webkit5-dev  libqt5svg5-dev pkg-config  cmake qt5-qmake libqt5svg5-dev git libsignal-protocol-c-dev libssl-dev libtidy-dev libxss-dev libotr5-dev

Download and build psi+ from source code

$ git clone --depth=1 https://github.com/psi-plus/psi-plus-snapshots 
$ mkdir psi-plus-snapshots/build
$ cd psi-plus-snapshots/build
$ cmake -DCHAT_TYPE=basic .. # or ../configure --qtselect=5 --disable-keychain
$ make -j5
$ sudo make install 
$ cd ../../
$ rm -rf psi-plus-snapshots # remove not needed anymore source directory

Just use downloader plugin. For manual install take a look at https://github.com/psi-im/resources

Enjoy

$ psi-plus