How do you fatten beef cattle by using hormones?

You cannot possibly fatten a beefer by hormone implant alone. Implants are only there to help you in achieving better feed effiency and growth in your finisher cattle, not to fatten them up period. Diet is also crucial in achieving level of fattening and time to which you slaughter your animals.

A small capsule containing hormones for that animal (there's different ones according to what gender the animal is and what you are trying to achieve in the fattening process) is injected into the ear under the skin of the steer or heifer. Usually one to three capsules is injected per animal: actual number of capsules to inject depends on the animal's body weight. The capsule along with its contents is absorbed into the animal's system and helps increase rate of gain or average daily gain and help with feed efficiency of that animal. In other Words, the hormones act to help the animal gain weight faster and convert feed more efficiently into muscle and fat. It would likely increase the animal's appetite as well.

Typically these hormones only last for 100 days, which means that these animals are not injected with hormones every day. The process to finish a steer or heifer inconsequentially also lasts for around 100 days or more.