How do I convert a 4-wire towing wire system to a 7-wire system?

I have a Ford Ranger with the towing package. It has a 4-wire flat connector. I bought a trailer with a 7-blade round connector.How do I convert a 4-wire towing wire system to a 7-wire system?
Just buy the adapter and plug it in.How do I convert a 4-wire towing wire system to a 7-wire system?
the seven one is for electric brake and you need to get the brake controller that goes in th vehicle i have the same got a adapter and my trailer recommends a controller and i don't use it cause it weight is under 1400 lb so you can just get a four to seven at any trailer supply store and wall mart believe it or not Carry's it as well as napa pep boys and auto zone its about 25 to 30 dollars if not let me know and i will sell you mine, but you trailer may recommend the use of the electric brake so make sure cause you do want to stop