The Graduale Romanum gives the following propers for St Stephen:

IN: Etenim sederunt principes (GR 632)
GR: Sederunt principes (GR 633)
AL: Video caelos apertos (GR 634)
OF: Elegerunt Apostoli Stephanum (GR 634)
ad libitum: In virtute tua (GR 512)
CO: Video caelos apertos (GR 635)

The Lumen Christi Missal has the English propers from the Roman Missal set to simple psalmtones (p. 66), as well as the responsorial psalm.

The Plainchant Gradual (1965, by G. Burgess and F. Palmer) has all the English propers (pp. 30-33).

The Plainsong Propers (1966, by the the Sisters of the Most Precious Blood, O'Fallon Missouri) has the following English propers for the feast of St Stephen:

By the way, should I be mixing Christmas Carols into this liturgy? I am looking at using "What Child is This?" and maybe "Silent Night"

I have chosen a Hymn for the liturgy for St Stephen and will try to have the propers sung.

Hymn “By All Your Saints Still Striving” (Aurelia) By Horatio Bolton Nelson

By all your saints still striving, for all your saints at rest
Your Holy Name, Lord Jesus forevermore be blest!
You rose, our king, victorious that they might wear the crown
and ever shine in splendour, reflected from your throne.

Saint Stephen:
Praise for the first of Martyrs, who saw Thee ready stand,
To aid in midst of torment, to plead at god's right hand.
Share, we with him, if summon'd by death our Lord to own,
On earth the faithful witness, in heaven the martyr-crown.

Apostles, Prophets, Martyrs, and all the sacred throng
Who wear the spotless raiment, who raise the ceaseless song;
For these, pass'd on before us, Saviour, we Thee adore,
And, walking in their footsteps, would serve Thee more and more.

Then let us praise the Father, and worship God the Son,
And sing to God the Spirit, Eternal Three in One;
Till all the ransom'd number bow down before the Throne,
And honour, power, and glory ascribe to God alone.

A hymn in LM http://www.chantcafe.com/2012/12/a-hymn-for-day.html

Try Eisenach