TLDR

If that documentation is too long, here's a lightning walkthrough:

Create a working environment and a project

cd /path/to/workarea
mkdir -p /path/to/workarea/run /path/to/workarea/build
cd build
setupATLAS # if you haven't done this already
asetup 21.2.32,AnalysisBase
lsetup git
cd ../
### choose a name for your project, I'm calling it "AnaProject"
curl https://gitlab.cern.ch/atlas-trt-software/TRTFramework/raw/v2.X-branch/scripts/GenerateProject.py | python - AnaProject

You now have a git repository with TRTFramework as a submodule. Let's compile it

cd build
cmake ../AnaProject ## or whatever you called your project
make -j4
source $AnalysisBase_PLATFORM/setup.sh
## if using a local cluster $AnalysisBase_PLATFORM might not be set
## you should source the setup.sh file which is in a folder that should
## start with x86_64...

Run your first job by copying over the example config file and sample list:

cd /path/to/workarea/run
cp ../AnaProject/TRTFramework/data/default.cfg .
cp ../AnaProject/TRTFramework/data/sample_list.txt .

Now run with

runAnaProjectLooperAlg -c default.cfg -i sample_list.txt -o job_output

Now go ahead and check out the algorithm code in AnaProject/AnaProjectLooper, that's where you'll need to do your development. The class AnaProjectLooperAlg inherits from xTRT::Algorithm which itself inherits from the EventLoop algorithm class (EL::Algorithm). Go read the config section and the doxygen documentation.