The disciples were sent to baptize people, but they were not sent to baptize people in the Holy Spirit. Jesus baptizes people in the Holy Spirit. John said that He Who came after Him would baptize with the Holy Spirit and fire. He did not say this of the twelve apostles.
So how did the apostles baptize? Look in Acts 10-11 when Peter first preached to Gentiles. He asked 'who should forbid water...' Let us look in this passage in Acts 10:
47 Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we?
48 And he commanded them to be baptized in the name of the Lord. Then prayed they him to tarry certain days.
Notice here that baptism was done in water. Also notice that Peter ___commanded___ that they be baptized. He did not have to perform all the washings himself. That is something that could be delegated to others in the church.
Paul in I Corinthians 1, was thankful he had baptized only Crispus, Gaius, and the household of Stephanus, and he did not remember if he had baptized any other. But if we read in Acts, many in Corinth were baptized. So someone was baptizing them. When we read that Paul said he was not sent to baptize, but the preach the Gospel, we need to realize that someone or some individuals were baptizing the other people. It could have been one of Paul's co-workers, but it is also quite likely that he had members of the new congregation do the baptizing. Like Peter commanded that the people be baptized, Paul could have let others do the baptizing in water. They were indeed baptized.
And we should assume he is talking about water here. If you acknowledge that he was talking about water when he said he wasn't sent to baptize, then you should acknowledge that he still baptized Crispus, Gaius, and the household of Stephanus (possibly the first converts and the first household) and the others still were baptized.
Is there any reason to think that hear in Acts 22 that this brother who baptized Paul was not using water:
14 And he said, The God of our fathers hath chosen thee, that thou shouldest know his will, and see that Just One, and shouldest hear the voice of his mouth.
15 For thou shalt be his witness unto all men of what thou hast seen and heard.
16 And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord.
The Jews in his audience would have been familiar with ceremonial washing in water. It is unlikely that they were familiar with metaphorical uses of the word. He quite possibly he spoke, as he spoke in Hebrew, of the mikveh, something Jews would have been familiar with.
In Acts 8, notice that Philip and the eunuch came up out of the water after Philip baptized him. It makes sense that Philip would have taught the eunuch something about baptism while explaining the Gospel to him for the eunuch to think about baptism when he saw the water.
Acts 8
35 Then Philip opened his mouth, and began at the same scripture, and preached unto him Jesus.
36 And as they went on their way, they came unto a certain water: and the eunuch said, See, here is water; what doth hinder me to be baptized?
37 And Philip said, If thou believest with all thine heart, thou mayest. And he answered and said, I believe that Jesus Christ is the Son of God.
38 And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him.
39 And when they were come up out of the water, the Spirit of the Lord caught away Philip, that the eunuch saw him no more: and he went on his way rejoicing.