Usually by taking the patella ligament from the front of the knee and using that to reconnect the 2 ends of the ruptured ACL. That's how mine was fixed in 1997 and that's how Gundogan had his done last year.
Actually 3 types of reconstruction: patella, hamstring or from a donor. Patella has generally been the favoured method for young athletes, but in certain (fairly rare) circumstances, can lead to longer term complications. For that reason, hamstring is considered to be the strongest by some surgeons, although the recovery is more difficult