Both of these methods are only used when they are considered to be the safest option for delivery, both for you and your baby.
Your midwife or obstetrician will explain whatever the reasons are for recommending an assisted birth, along with going through the procedure with you and their choice of instruments.
They will also explain any risks that are involved for either your baby or you during the process of an assisted birth. If the delivery of your baby is taking place in an operating theatre, then the medical team will obtain your written consent. Otherwise, they will ask for your verbal consent.