Install yum-utils 

yum install yum-utils createrepo

Create a directory for the repo 

mkdir /my_repo

Get the RPM's to put in the repo. Use reposync to download from an existing repo or download rpm's manaully.. e.g. 

wget http://mirror.lihnidos.org/CentOS/6/os/i386/Packages/NetworkManager-0.8.1-43.el6.i686.rpm

Create the repository 

createrepo /my_repo

Create /etc/yum.repos.d/custom.repo with the following contents 

[customrepo]

name=Custom Repository
baseurl=file:////my_repo/
enabled=1
gpgcheck=0