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

Adding of special PPA:

1) echo "deb https://notesalexp.org/debian/buster/ buster main" | sudo tee /etc/apt/sources.list.d/psi-plus.list
2) sudo nano /etc/apt/preferences

Package: *
Pin: origin notesalexp.org
Pin-Priority: 1

Package: psi-plus*
Pin: origin notesalexp.org
Pin-Priority: 900

3) sudo apt update --allow-unauthenticated
4) sudo apt install notesalexp-keyring --allow-unauthenticated -y
5) sudo apt update
6) sudo apt install psi-plus-webkit psi-plus-plugins psi-plus-l10n psi-plus-sounds psi-plus-skins

In the future you will receive updates of psi-plus* packages as usual:

sudo apt update
sudo apt upgrade

Note, Psi+ won't build with Qt version < 5.6.0

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

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