I'll sing you one, O
Green grow the rushes, O
What is your ___, O
One is one and all alone and evermore shall be so.

The part in bold repeats for every verse. You add on each number, as in The Twelve Days of Christmas. To get you started, I'll type in a couple more verses exactly as we sing them.

I'll sing you two, O Green grow the rushes, O
What is your two, O
Two, two, lily-white boys, clothed all in green, O
One is one and all alone and evermore shall be so.

I'll sing you three, O
Green grow the rushes, O
What is your three, O
Three, three, the rivals
Two, two, lily-white boys, clothed all in green, O
One is one and all alone and evermore shall be so.

I'll sing you four, O
Green grow the rushes, O
What is your four, O
Four for the gospel makers
Three, three, the rivals
Two, two, lily-white boys, clothed all in green, O
One is one and all alone and evermore shall be so.

...and the rest continues in the same pattern. I hope this helps. If it doesn't, please let me know.