It doesn't really say how they got to earth at all really, they just kind of show up in halo 3 on earth with no explanation, even though chief left and arrived at earth before them. Supposedly they were taken to Earth by an elite ship. In the comments above, LoA says that chief got to Earth after them because Truth knew Chief was on the forerunner ship and ordered it to stay in orbit around Mars, at which point chief jumped off while going over Earth.