I'm not too good at arranging flowers. The bride wants me to take some loose roses and make them into a bouquet and place it on top of the cake (like the picture). Can I use florist tape to put the flowers together. I have never made a bouquet before and haven't a clue where to start.

There are several ways to do this. You can certainly use florist tape.

I usually find a smal plastic dish or a container lid (white or clear) and use a bit of floral foam (0asis) to hold the flowers in place. You can wet the floral foam, but generally, I don't need to.

If you are going to serve the top, you can also insert straws into the cake to hold the floral stems in place. It will leave holes, so if they want an anniversary tier, It would look odd, unless they put in new flowers for their anniversary.

I think she just wants the bouquet laid on top of the cake.
I just don't know how to make the bouquet.