Primary source: Ordo Romanus primus, E. G. Atchley, 1905
Secondary source: The Mass of the Western Rites, Dom Fernand Cabrol (chapter 4, "The Mass at Rome, from the Fifth to the Seventh Centuries")
Without further ado:
THE EASTER SUNDAY MASS IN THE CITY OF ROME AT THE TIME OF ORDO ROMANUS I
I. THE ENTRANCE PROCESSION
At daybreak, Acolytes of the district (in this case, that of the 3rd Ecclesiastical District of Rome) and the Counsellors (Defensores) of every District meet at the Lateran Palace and go before the Pope, who rides on horseback to the Stational Church (Lay grooms walk beside the Pope should the Pope's horse stumble).
St. Mary Major.
-The Papal Vicar, the Sacristan, the Invitationer and the Treasurer ride behind him.
-The Stational Acolyte walks in front of the Pope, carrying an Ampulla filled with Chrism wrapped in a napkin.
-The other Acolytes also walk, also carrying Chrism and other necessary stuff such as linen cloths, sacks, Gospel books and basins (for hand-washing).
-Bearers carry the vessels, such as the daily Paten, the Chalice, the Reeds (also known as fistula; these are straw-like devices made of gold or silver which was used to drink the Precious Blood. This usage survived in the pre-Conciliar Papal Mass), the silver Gemellions (water basins), the strainer made of gold and silver (and a more larger one made of silver), the Flagons, the Gradual-Book, the Candlesticks and the rest which was taken by the Chief Sexton from the Lateran Basilica. (On greater Feasts, the larger and gem-studded Paten and Chalice from the Papal Vestry are brought and used; these being protected by the Sacristan's Seal to prevent theft.)
-The Lay Chamberlain goes ahead of the procession to prepare the Pope's Chair in the Sacristy.
On the way to St. Mary Major (the Station Church for Easter Sunday), the district-Notary stands in "Ad Merulanas" (the present-day Via Merulana), and after saluting the Pope, says to him:
"In nomine Domini nostri Jesu Christi, baptizati sunt in sancta Dei Genetrice Maria infantes masculi numero tanti feminae tantae."
(In the name of our Lord Jesus Christ, last night there were baptized in the Church of St. Mary the Mother of God so many baby boys, and so many baby girls.)
To which the Pope replies, "Deo gratias." (Thanks be to God.)
Via Merulana (Santa Maria Maggiore is visible on the upper middle).
Then the Notary receives a solidus from the Treasurer as the Pope goes on and makes his way.
Those who were not part of the Procession make their way into the Church to meet the Pope, "with the Papal Almoner and the bearers and the rest who carry crosses, sitting in the presbytery; the Bishops, that is, on the left hand as they enter, the presbyters on the other hand on the right, so that when the Pontiff sits down and looks towards them, he may see the Bishops on his right hand and the presbyters on his left."
When the Pope is finally near the Church, the Acolytes and Counsellors of the District, the Presbyter of the title or the Church where the Mass is to be held, the Majordomos of the Roman Church, or the father of the hostelry (patre diaconiae; if that church happened to have one) and the Sexton humbly wait for him at the appointed spot. The Sexton carries a Censer out of respect for the Pope.
When the Pope finally arrives, those waiting will all bow their heads. The Acolytes with the Counsellors will separate into groups on either side and make their way before the Pontiff to the Church; the Advocates (Advocatores) with the Major-domos follow the Pope's horse, together with the Acolyte who carries the Basin.
A. THE ECCLESIASTICAL DISTRICTS
Back then, Rome is divided into Seven Ecclesiastical districts, with a Deacon alloted in each. The Acolytes of a particular district are under the district-Deacon's jurisdiction, save when he dies; in that case the Acolytes are subject to the Archdeacon (the Pope's Vicar) until another Deacon takes the former's place.
These Seven districts are each assigned their own days of the Week:
(Easter) Sunday-3rd District
Saturday (aka Sabbath)-2nd District
B. STATIONAL CHURCHES
Station churches are the churches that are appointed for special morning and evening services during Lent, Easter and some other important days. The tradition started in order to strengthen the sense of community within the Church in Rome, as this system meant that the Holy Father would visit each part of the city and celebrate Mass with the congregation.
Read more about Stational Churches here: