It was a difficult command. The Lord told his disciples to preach but not to bring anything for their journey. Nothing.
But the apostles obeyed the Lord. They went and preached, with nothing in their purse, no sandals on their feet.
Obedience is best learned in moments of difficulties and crisis. In times of grief and sorrow. If we are still able to say, "Lord, let your will be done" in moments of pain and grief, then that is true obedience.