Question
How to insert calla lilles in wet foam bouquet holder
without breaking the stems?

Answer
Hi May,

You want to wire and tape the stems first.  Take floral wire, inset through the stem about 1/8th of an inch below the bloom, wrap the wire around the length and then cover with fabric floral tape wrapped over that.  Cut the stem, along with the wire and tape, at a 45 degree angle.  For particularly soft stemmed floral material, you might also want to give the oasis a "pilot hole" to make insertion easier.