How was baptism done: immersion or pouring, and with what words?

Didache c. 100 ADDidache 7.1–4

Concerning baptism, baptize this way: having first said all these things, baptize in the name of the Father and of the Son and of the Holy Spirit, in living water. If you do not have living water, baptize in other water; if you cannot in cold, then in warm. If you have neither, pour water on the head three times in the name of Father and Son and Holy Spirit. Before the baptism, let the one baptizing and the one baptized fast, and any others who can.

"Living" water is running water. Pouring is allowed when there is not enough water. The words are the triple name of Matthew 28:19.

Tertullian c. 155–c. 220 ADOn the Crown 3.2–3

When we are about to enter the water, there, and also a little earlier in the church under the bishop's hand, we declare that we renounce the devil, his pomp and his angels. Then we are immersed three times, answering somewhat more than the Lord laid down in the Gospel. Then, taken up, we taste a mixture of milk and honey, and from that day we abstain from our daily bath for a whole week.

Tertullian lists customs kept by tradition without a written text. "Answering" refers to the questions on the creed put at each immersion.

Cyprian of Carthage c. 200–258 ADLetter 69, to Magnus 69.12

You also asked, dear son, what I think of those who receive God's grace in sickness and weakness: whether they are to be counted true Christians, since they were not washed with the saving water but had it poured over them. ... As far as my small judgment reaches, I think that God's gifts cannot be cut short or weakened in any way, nor can less happen there, where what is drawn from God's gifts is received with full and whole faith by giver and receiver.

"Clinical" baptism, by pouring on the sick in bed. Cyprian accepts it as full baptism.

Cyril of Jerusalem c. 313–386 ADMystagogic Catecheses 2.4

And each of you was asked whether he believed in the name of the Father, and of the Son, and of the Holy Spirit. And you made the saving confession, and went down three times into the water and came up again, signifying by a symbol the three days of Christ's burial.

The Eastern churches still baptize by triple immersion.