What instructions did Jesus give His disciples before He returned to Heaven?

In his Ascension homily in 1979, Pope St. John Paul II said,

In the Scripture readings the whole significance of Christ’s Ascension is summarized for us. The richness of this mystery is spelled out in two statements: Jesus gave instructions, and then Jesus took his place. …
The instructions indicated, above all, that the Apostles were to wait for the Holy Spirit, who was the gift of the Father. From the beginning, it had to be crystal-clear that the source of the Apostles’ strength is the Holy Spirit. It is the Holy Spirit who guides the Church in the way of truth; the Gospel is to spread through the power of God, and not by means of human wisdom or strength.
The Apostles, moreover, were instructed to teach – to proclaim the Good News to the whole world. And they were to baptize in the name of the Father, and of the Son, and of the Holy Spirit. Like Jesus, they were to speak explicitly about the Kingdom of God and about salvation. The Apostles were to give witness to Christ to the ends of the earth. The early Church clearly understood these instructions and the missionary era began. And everybody knew that this missionary era could never end until the same Jesus, who went up to Heaven, would come back again.