We started with A5-sized sheets of paper for both the vases and the roses.
Making the vases first: you twist the sheet of paper like this.
Then tighten until it forms the vase shape with a point on the bottom end.
We just taped of ours at the back because no one will see the backs for ours, but if you wanted it to be neater you could use a little glue.
To make the roses (I used relatively thick paper so that it would be less frustrating for little hands in terms of tearing while twisting, but to get a finer looking rose a softer/thinner paper, like crepe or tissue paper, would probably work nicely): lay the paper down, roll it around one finger, like this.
Then roll it again, but this time all the way around two fingers.
Once the paper is completely rolled around, squeeze in along the bottom of the "rose" and twist in the opposite direction to make a stem.
Put tape on the flower stem and then insert into the vase, push down on the tape to stick it in the vase. I found it was easier to put the tape on before putting the rose into the vase, because once inside it is hard to move your fingers around in the little vase. I actually stuck a pencil inside the vase to push down on the tape, to make sure it stuck down properly.
To finish off, use a hole punch to make a hole at the top of the vase and add ribbon or string to hang it. If you prefer, you can also skip this stage and just stick them directly onto the wall with tape or sticky putty (for those in South Africa - a.k.a prestick!).