As it seems to be you are from maharashtra, thus for registration as principle employer for RC you can go to below given link and make an application. Complete process for application is readily available on the site which you can follow and do the needful.
Secondly you need to make sure that, the contractors hired by you are following all statutory norms. As you have mentioned that, the employees working under contractor's are not regular employees thus contractor may face trouble while paying PF and other statutory dues which may cause for violation in statutory procedures.
20th January 2018