Select CentOS-7-x86_64-NetInstall-2009.iso
Depending of you systems burn it on CD or create a bootable USB disk.
Of course not needed when working with virtual machines
Boot the system
In GRUB menu, select Install CentOS 7
Select Language
Adapt the installation summary as you want.
Setup network and hostname (really important)
Setup Date and Time
Language Support
Select Keyboard layout
CENTOS NetInstall URL (Installation Source)
http://ftp.belnet.be/mirror/ftp.centos.org/7.9.2009/os/x86_64/
http://ftp.belnet.be/mirror/ftp.centos.org/7/os/x86_64/
Continue the installation as always....