First create a package_name.spec file with the following content:
Summary: Summary here
Name: package_name
Version: 0
Release: 0
License: Public
Group: Applications/System
Requires: libXrandr.i686,gtk2.i686,libXtst.i686
%description
Package description here
%files
After this file is created simply run # rpmbuild -bb package_name.spec
to create the package.
You don't really need Requires unless you want the rpm to depend on other rpms.