Lethal injection is the practice of injecting a person with a fatal dose of drugs, typically a barbiturate to render the person into a deep sleep, Paralytic to paralyze the lungs and diaphragm, and potassium chloride to induce cardiac arrest. The first drug falls under the category of an anesthetic and is used in a large enough dose that it is considered fatal on it's own. It is believed that after the first dose most persons fall into death, and cannot feel the two subsequent injections to follow, therefore it is more humane than the electric chair.
Teresa was probably moved to death watch area a few days before the execution to isolate her from the death row community and prepare her for the execution. In the last twenty-four hours she would have been visited by clergy, family, and the warden. Attorneys would have been able to update her periodically about the status of a possible stay of execution. Then her last meal would have been prepared as requested. Most prisons will do their best to prepare any meal the prisoner requests for the last meal. As the witnesses were being brought into the execution chamber's viewing area Teresa would be hooked up to a heart monitor, this will allow the medical examiner to pronounce death following cardiac arrest.
Being that no stay of execution was ordered she would now be escorted to the execution chamber and strapped to the gurney in that chamber by three to four leather straps. She may have been offered a sheet for comfort. Now they would insert two to three intravenous lines in one or both arms. The tubes go through the wall into what they call an "anteroom". This is where the executioner, the guard who will actually administer the fatal doses, will be located. Phones are on the wall in this room so the warden can stop the execution at any time, perhaps for a last minute stay of execution. Unfortunately this did not happen for Teresa.
The curtains are pulled back so the prisoner can view her witnesses and they can view her. Teresa asked if her deceased husband's daughter was there and stated "I want Kathy to know that I love her and I am sorry". Those were her last words and the execution proceeded. Witnesses stated it was very peaceful with only small movement of her feet noted after the first injection. She was Pronounced dead at 9:13 pm and her body then released to the medical examiner.
Sources:
Wikipedia.com
"How Lethal Injection Works", author Kevin Bonsor @ How stuff works.com
"How Lethal Injection Works", author Kevin Bonsor @ How stuff works.com

