There is no formula to follow, this is your prayer, your conversation with God. What do you need to pray for? This is a time to ask for wisdom, courage, grace, consolation, and healing from the Holy Spirit, but once again, this is your conversation with God, so what do you need to seek from the coming of of Holy Spirit?
Every year, I like to make my daily novena and to listen to Veni Sancte Spiritus from Taize, which I include here. I love the repetitive chant, seeking the coming of the Holy Spirit, the sound of which typically brings me to a place of silence and peace.
Whatever your prayer to the coming of the Holy Spirit might be, find the words of your heart, which may simply be silent waiting, and offer them to God these nine days. Come Holy Spirit, come.