There are no hard and fast rules - you need to take it at their pace and to the actions they are comfortable with. Sitting by their cage and talking to them and offering food from your hand is usually a good first step - once they take food from you then that is usually a sign of trust. After that moving onto trying to handle them by herding into a carrier first
But as I say taking it at their pace - sitting by their cage and having them take food from your hand may be as much as they ever want; or they may enjoy a lap time snuggle given time - or anything in between!
I personally prefer to sit in their pen and let them come to me as and when they are ready. My piggies did not like being handled, so I saw no reason in attempting to do so as it was stressful for all of us - so instead sitting with them and allowing them to come to me worked so much better. They learnt to trust me and eventually were happy to be around me - although, like most piggies, they still don’t want to be handled (handling is kept brief and to health and weight checks)