How do I build thigh muscle after having ACL surgery?

Slowly. Patience is crucial. Start with very light weight or just the weight of your own leg. Try 8-15 repetitions per exercise. Very slowly and gradually increase your number of reps over the course of the first 4 to 8 weeks of training. After that, if all is well, consider increasing the reps and sets for another month. If all goes well after that, you can start increasing the weight in very small increments, always giving your body a week to adapt to weight increases before increasing the weight further. Use perfect technique or you risk re-injury.