Plenary Indulgence on January 1st (tomorrow) by reciting the prayer/hymn
Come Holy Spirit, Creator Blest

Come Holy Spirit, Creator Blest
(Veni, Creator Spiritus)

Come, Holy Spirit, Creator blest,
and in our souls take up Thy rest;
come with Thy grace and heavenly aid
to fill the hearts which Thou hast made.
O comforter, to Thee we cry,
O heavenly gift of God Most High,
O fount of life and fire of love,
and sweet anointing from above.

Thou in Thy sevenfold gifts are known;
Thou, finger of God’s hand we own;
Thou, promise of the Father, Thou
Who dost the tongue with power imbue.

Kindle our sense from above,
and make our hearts o’erflow with love;
with patience firm and virtue high
the weakness of our flesh supply.

Far from us drive the foe we dread,
and grant us Thy peace instead;
so shall we not, with Thee for guide,
turn from the path of life aside.

Oh, may Thy grace on us bestow
the Father and the Son to know;
and Thee, through endless times confessed,
of both the eternal Spirit blest.

Now to the Father and the Son,
Who rose from death, be glory given,
with Thou, O Holy Comforter,
henceforth by all in earth and heaven. Amen.

[from Catholic Online Prayers]

Recall the usual conditions for a plenary (full) indulgence:
https://www.catholic.org/prayers/indulgw.php