Summarize:

While still holding your index fingers together in a steeple formation, swing your thumbs apart. Remember that your thumbs are functioning as the door of the church, so opening them emulates opening the doors of the church. Keep your other fingers locked as you do this. Tilt your wrists slightly upward to show the locked fingers inside your hands. Your locked fingers will be lined up as if they are the people sitting inside pews inside the church. They represent the people inside the church, who are revealed once you open the thumb doors. It's more fun for children to wriggle their fingers to show that the people are moving inside the church. You're ready now to say the final part of the rhyme: "Open the doors, and see all the people." Moving your locked fingers adds a colorful touch to the "people." Many people drop the last part of the nursery rhyme: "Here's the parson going upstairs. And here he is saying his prayers." A parson is a member of the clergy, especially Anglican. However, you can add the words at the end if you want. If you do, end your hands with them pressed together in prayer. Press both hands together, with the thumbs pressed together and the fingers point toward the sky. The rhyme is thus a good way to get children to start to pray. The hands in prayer formation are meant to represent the parson praying. If you're secular, you can drop the last line and prayer hands and just end on the wriggling fingers inside, representing the people.
Open your thumbs. Wriggle the fingers. Say the last part of the rhyme.