FAIRsoft should be installed (see PandaRoot)
Or follow this description:

Install the dependencies
Debian, Ubuntu and other Debian based systems (64bit):

$ apt-get install  cmake cmake-data g++ gcc gfortran \
  debianutils build-essential make patch sed \
  libx11-dev libxft-dev libxext-dev libxpm-dev libxmu-dev \
  libglu1-mesa-dev libgl1-mesa-dev \
  libncurses5-dev curl libcurl4-openssl-dev bzip2 libbz2-dev gzip unzip tar \
  subversion git xutils-dev flex bison lsb-release python3-dev \
  libc6-dev-i386 libxml2-dev wget libssl-dev libkrb5-dev \
  automake autoconf libtool

sudo apt-get install libxi-dev

Clone the Fairsoft package
e.g. to your home directory /home/username

$ git clone https://github.com/FairRootGroup/FairSoft fairsoft_may18
Cloning into 'fairsoft_may18'...
remote: Counting objects: 1385, done.
remote: Total 1385 (delta 0), reused 0 (delta 0), pack-reused 1385
Receiving objects: 100% (1385/1385), 17.86 MiB | 4.03 MiB/s, done.
Resolving deltas: 100% (904/904), done.
Checking connectivity... done.

$ cd fairsoft_may18
$ git checkout tags/may18p1

Launch the ./configure.sh script¹⁾
and answer to the questions as the following:

1. GCC (Linux)
2. Optimize with Debug Info
3. Would you like to install FairMQ only? - No
4. Would you like to install Simulation engines and event generators? - Yes
5. Would you like to install the additionally available data files the GEANT4 package? - Internet
6. Would you like to install the python bindings for Geant4 and Root? - Yes
7. Please define a directory for the installation of the external packages. - /home/username/fairsoft_may18/build
8. Is /home/username/fairsoft_may18/build the correct path? -Yes

Now you can start doing something else, since the installation will take some time. At the end, you should have the following message:

**** Checking libraries in /home/pandauser/fairsoft_jun19/build/lib ****
*** End installation of external packages without Errors***

In case some error has occurred try it again. If it is still not working ask for help.

git clone agmaas@bwec-fileserver:/home/agmaas/git/primasoft.git

since 2020:

git clone https://gitlab.rlp.net/emp/primasoft.git

cd primasoft/

Define the environment variable FAIRSOFT_BUILD_DIR and set it equal the path to the FAIRsoft build directory

export FAIRSOFT_BUILD_DIR=/path/to/fairsoft/build
source setenv.sh

cd geant4/
source $FAIRSOFT_BUILD_DIR/share/Geant4/geant4make/geant4make.sh
make

geant4$ primag4sim primag4sim.conf

In order to execute rootmacros, the include paths need to be set.
This can be done automatically with a rootlogon.C file.
Create the rootlogon.C file:

$ cd ~/
$ mkdir .rootlogon
$ geany .rootlogon/rootlogon.C

Enter the following lines to your rootlogon.C file:

// This is the file rootlogon.C                                                 
{
  printf("\n Beginning new ROOT session\n");

  gROOT->ProcessLine(".include your_path/primasoft/geant4");
  gSystem->Load("<geant4_workdir>/tmp/Linux-g++/primag4sim/libprimadict.so");                              
}

Here, your_path is the path to the primasoft git repository and the <geant4_workdir> can be found out by $ echo $G4WORKDIR.

Now root needs to know where to find the rootlogon.C file. Therefore in ''$FAIRSOFT_BUILD_DIR'/share/root/etc/system.rootrc' change the line:

Rint.Logon:    rootlogon.C

to:

Rint.Logon:    ~/.rootlogon/rootlogon.C

Now you can execute the rootmacro:

/geant4$ root -l rootmacros/test.C+