The Synoptic Gospels and Paul give us the words and actions of Jesus at the Last Supper with some variations. John does not have anything similar at the Last Supper, but he gives us a profound Eucharistic teaching in Chapter 6.
These accounts are set in a framework of Jesus' life, including his many meals with his disciples, his suffering, death and resurrection.
Paul in his first letter to the Corinthians (11:17ff) scolds them for being in such a hurry to eat their fill that they leave the poor out of the meal in which they celebrate the Lord's Supper. So from this, we know that in the very early Church, the Lord's Supper was celebrated at a meal as Jesus did at the Last Supper.
Met in their homes
Acts (2:46-47) also tells us that they met in their homes for the breaking of the bread, sharing their food and praising God.
We do have a number of early texts that speak specifically of the prayers and manner of conducting the Lord's Supper. These texts come from several regions of the early Christian world and are necessarily influenced by their social and religious milieu. Often, early texts are those written by Christians in their own defence as they were seen as suspect by both pagans and Jews because they separated themselves from public cult and held their own private gatherings.
Justin who lived about 100-165 AD, gives us a text, written in the middle of the second century, that is most pertinent to the present question. It is in much the same pattern as our Mass today. He tells us that on Sunday, they come together and "the memoirs of the apostles or the writings of the prophets are read as long as time permits," which is then followed by the presider's discourse urging imitation of the models presented in the readings.
After this, all stand up together to offer prayers and then "bread is brought and wine and water and the president sends up prayers and thanksgiving, and the congregation assents, saying the Amen." Then the "distribution and the reception of the consecrated elements by each one takes place and they are sent to the absent."A collection is taken to be used to take care of the needy.
Justin tells us the meaning of this celebration: "We do not receive these things as common bread or common drink, but as Jesus Christ our Saviour being incarnated by God's word. The food consecrated by the word of prayer is the flesh and blood of that incarnate Jesus."
He also specifies the conditions for partaking of the Eucharist: "One who believes the things we teach are true and has received the washing for forgiveness of sins and who lives as Christ handed down to us."
Another appropriate text is that of Hippolytus in the apostolic tradition, which gives us the wording of a preface and Eucharistic prayer from the beginning of the third century. We see here the model for our present-day second Eucharistic prayer and very similar to others we use.
The bishop and the priests lay their hands on the offering, saying: "The Lord be with you" to which the people respond "And with your spirit." Then they continue: "Lift up your hearts." "We have them with the Lord." "Let us give thanks unto the Lord." "It is meet and right." Then the presider continues with the preface similar to our prefaces.
This is followed by the Eucharistic Prayer: "We give you thanks, O God, through your beloved child Jesus Christ" and the words of Consecration: "Taking bread and making Eucharist to you, he said, 'Take, eat, this is my body which is broken for you.' Likewise also the cup, saying 'This is my blood which is shed for you. When you do this you make memory of me.'"
After the words of Consecration, he continues with the remainder of the Eucharistic prayer.
This text goes on to remind us: "It is not altogether necessary to recite the very same words. Let each one pray according to his own ability. But should he recite a prayer according to a fixed form, no one shall prevent him. Only let his prayer be correct and right in doctrine."
So you can see that, in essence, the meaning of the Eucharist, the form and the words used for its celebration, as well as the requirements for the reception of Communion have remained intact through the centuries. While following the form and the words, we should also enter fully into the meaning of the Mass by being open to God's presence and allowing God to transform our hearts and minds.