Indonesia is set soon to execute eight foreigners convicted of drug offences, despite international outrage and desperate appeals for mercy from relatives.

Authorities on Saturday gave formal notice to the eight -- from Australia, Brazil, Nigeria and the Philippines -- that they would face a firing squad imminently.

If the executions go ahead, the seven men and one woman will be led to a clearing on a prison island to be shot by police marksmen -- after authorities have placed a black mark on their clothing to mark the heart.

Here are key questions and answers about the process:

Q: Where do the executions take place?

A: Indonesia executes death row inmates at Nusakambangan, a rugged island off Central Java that has served as a high-security jail since Dutch colonial rule.

Among the more notorious inmates to spend their final days there were Imam Samudra, Mukhlas and Amrozi, three men behind the 2002 bombings on the resort island of Bali that killed 202 people. They were put to death there in 2008.

Those sentenced to death are eventually transferred to Nusakambangan, where they wait until being led to a nearby clearing to face the firing squad.

Q: How much notification do prisoners receive before being shot?

A: Authorities must provide a minimum 72 hours' notice. Once this is given, the prisoners are moved to isolation cells to wait. If they are foreigners their governments are informed of the impending execution.

Q: What happens next?

A: One hour before the scheduled execution time, a team of 12 specially-trained policemen assembles at the site.

They take position five to ten metres (16 to 32 feet) from where the condemned inmates will be positioned and lay out their rifles.

A commander loads each rifle with one round but only three of the rounds are live. The rest are blanks, meaning it cannot be determined who fired the fatal shot.

Prisoners sentenced to death for the same crime -- like the two Australian ringleaders of the so-called "Bali Nine" heroin-smuggling gang -- must by law be executed at the same time but by a different shooting group.

Q: How are the prisoners executed?

A: The condemned inmates are marched to the clearing, where their hands and feet are bound and they are placed in front of individual posts. They are given the option of sitting, kneeling or standing and can wear a blindfold if desired.

The prisoners then have a final three minutes with a religious counsellor, before a commander draws a black mark on the inmate's clothing over the heart.

The squad commander then raises a sword. The marksmen take aim and fire when he swishes the blade down.

If a doctor determines the prisoner has survived, an officer delivers a single shot to the head by handgun.

Q: Do Indonesians support the death penalty for drug use?

In a nationwide survey last month by pollster Indo Barometer, 84 percent of respondents supported the death penalty for drug traffickers, while just 12 percent disagreed.

The broad public support partly explains why President Joko Widodo -- who believes Indonesia faces a drugs emergency -- has been so unwavering in his determination to put drug dealers to death.

Many in Indonesia view drug dealers as akin to terrorists, mass murderers or rapists.

The country has some of the world's toughest anti-drugs laws and sentences for possession of even minor quantities of narcotics can be harsh.