Using a 3.5mm hook and green double knitting yarn, you need to chain enough to cover the length of the pipe cleaner apart from just under 1cm at one end. The pipe cleaners I used are 16cm long, so I worked as follows (Note, I am English so give English instructions. An English dc is equivalent to a US sc.):
Make a slip-knot leaving sufficient tail end to sew the stem to the flower. Chain 31.
1dc into each chain, starting from the second chain from the hook (total 30dc).
Chain 1
Then work a row of dcs back down the opposite side by picking up the foundation chain loops.
When you get to the end
DO NOT make a chain, but dc into the first stitch of the original side.
This will curl the end over slightly for the end of the pipe cleaner to
sit in.
Work the rest of the row of dcs to the end.
Place the pipe cleaner on the crochet with one end tucked into the
curled up end. At the other end there should be just under 1cm left
clear.
Start to close the two sides together by slip stitching the stitches from each side together.
Work down the length of the pipe cleaner.
When you get to the end, slip stitch into the end to secure it, fasten off and darn this end in.
The completed flower stem, ready for the end to be darned in and showing the uncovered tip of the pipe cleaner.
Attach the other end to your flower, using the remaining length of tail. I was making daffodils, so the end of the pipe cleaner poked through the little hole in the centre of the flower and I stitched the stem to the back of the flower.