Couldn't find the methods used in practice. But presumably it's electrocution or CO/CO2 poisoning (see the allowed methods in https://eur-lex.europa.eu/legal-co

Couldn't find the methods used in practice. But presumably it's electrocution or CO/CO2 poisoning (see the allowed methods in https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A02009R1099-20191214 https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A...) or I might have been wrong and shreddering may indeed be used on its own, but can't be bothered to research further.