So you have an X server running with multiple displays (or monitors).
You are using the great arandr tool to quickly setup your setting. But, you don't want to do this each time you are starting your X.
What can you do?
First of all, hit the "Save As" button in you arandr gui and store the shell script.
Afterwards, adapt the file that is read once you login into your system or start your X. This can be .xinit, .login or .profile.
Inside this file, simple add the adapted content from below and thats it.
#maybe you have to change the path ".screenlayout" or the file name "default.sh" if [ -f $HOME'/.screenlayout/default.sh' ]; then . $HOME'/.screenlayout/default.sh' fi