After Changing The Brake Pads My Brake Pedal Goes To The Floor Changed Brake Pads In Front Of My 1997 Ford Taurus SHO And Now The Brake Pedal Goes All The Way Down?

Changed brake pads in front of my 1997 Ford taurus SHO and now the brake pedal goes all the way down? - after changing the brake pads my brake pedal goes to the floor

So my father changed the front brake pads on my car, then the car ran well, but after a while the pedal to the end of the floor. I went to open the rear brakes changed and the mechanic told me that I could be master cylinder or I should bleed the brakes. If I bleed the brakes, what is the right way to do it manually, I heard you have any tire not in order or somthing .......