I hope I can help, this was my method for putting fresh flowers on a cake:
1. Get some plastic throw-away champagne glasses. The kind that are in 2 pieces, the bottom base is removable.
2. Get some green Oasis (flower section of craft stores).
3. Soak the Oasis in water thoroughly and push the top of the glass into the wet Oasis. Then trim to have the top of the glass and Oasis even. '
4. Arrange your flowers by sticking the stems into the green foam.
5. Trim your stems so the flowers fit between your cake layers. You might need more than one glass, depending on the size of the flowers and the size of your layers.
7. Wash and dry the bottom stem thoroughly and insert the pointy stem of the glass into your cake.
(You might want to put a doily on the cake layer, to prevent the flowers from touching the icing)
8. No multiple holes - only one with @ 6 roses/flowers per arrangement.
9. Best part - Flowers can be arranged the day before and stay fresh because they are in water.
I hope this helps.