They got some practice while Jesus was alive. In Luke 10 Jesus sent out 72 of his followers to preach and to heal. (Perhaps Stephen had been one of the 72.) He sent them out two by two as his advance men to prepare the people where he would be going. You could say they went into the towns to stir up interest and get the people excited about Jesus' arrival. Jesus authorized them to do this. The disciples were operating under his authority. And in his name great things happened: not only were people healed but even the demons submitted to the name of Jesus.
What is the difference between the 72 who were sent out while Jesus was alive and the many disciples who went out after his death and resurrection? Jesus had told them to wait until they were baptized with the Holy Spirit (Acts 1:5). They received the power of the Holy Spirit at Pentecost and were authorized and empowered to pass that on to all who would come to believe. They are not acting solely under the deputed authority of Jesus, who was acting on the authority of his Father (John 5:19-23). The entire Trinity is now backing them up.
For reflection: If the Trinity has my back, what can I do to preach, to heal, to defeat demons, to bring the kingdom of God on earth?
Let us pray. Father, thank you for creating me in your image and likeness. Thank you for adopting me into your family through baptism. Thank you for saving me through Jesus' death on the cross and authorizing me to do your work with the power of the Holy Spirit. May I always be a good ambassador for you.