St. Thomas Aquinas tells us that a Spiritual Communion is "an ardent desire to receive Jesus in the Holy Sacrament" and an "ardent embrace [of Him] as though we had already received Him."

Those who are unable to make sacramental Communion with Christ by participating in the Eucharist, can express the desire to receive him in their spirit with these words that help to make spiritual communion with him:

My Jesus, I believe that you are truly present in the Most Holy Sacrament of the Altar. I love you above all things and desire to receive You into my soul. Since I cannot at this moment receive you sacramentally, come at least spiritually into my heart. I embrace You as if You were already there and unite myself wholly to You. Never permit me to be separated from You.
Amen. 

This prayer of Saint Alphonsus Liguori is the prayer recommended in the
Vatican prayerbook STRONG IN THE FACE OF TRIBULATION​​​​​​​