There are various liturgies for outdoor services, some of them approved within various churches e.g. the Church of England, and some of them not. These resources should help you:
One way to develop your own liturgy is to start with the people at church. Ask them “What Jesus songs/prayers/words do you know?”
Welcome / greeting
Here’s one that I created:
Person: We say to each other: “The Lord be with you. And also with you” All: Repeat
“We have welcomed ourselves: widened the circle to allow ourselves to feel at home here. Be yourself here, in love. Feel free to wander off, dance, sing. Express yourself to God.
We widen the circle to include a space for Jesus to speak to us at all times. [Make sure there is room in the gathering to include an invisible “extra space” for Jesus.]
We widen the circle to what creation wants to say to us. In Deuteronomy 3:27, Moses was told to go up the mountain and look west and north and south and east, to look at the land with his own eyes. This is the land that God will redeem, and that God wants to speak to us about his eternal power and divine nature. [We look west, north, south and east. We do an activity that helps us to look at this land. We ask: what can we learn about God from creation?]
We widen the circle to include people in history. Our words are drawn from 1000s of years of worship. We remember, O Lord, those who have brought us into the world, borne with us and guided us, sacrificed themselves for us, those who laid out hte fields, trod the roads, established this town and brought the Good News to us here. Remember, O Lord, the souls of all those who have kept the faith. Through Christ our Lord, Amen.