open a konsole and do the following:
1.cd /etc/init.d/
2.touch mumulache
3.vim mumulache
4.enter wpa_supplicant -D wired -i eth0 -c /etc/wpa_supplicant/domski_lan.conf -B in this file
5.save the file
6.chmod +x mumulache
7.chkconfig -a mumulache

The above commands does the following
1. changes directory to a directory which contains the startup scripts
2. creates a file named mumulache
3. edit the file mumulache (vim is a text editor....use man vim if you are not familiar with it)
4. write your command in this file
5. save the file (again....use man vim to learn how to save the file)
6. change the file to executable making it a script
7. add mumulache script to startup so it starts even after reboot