I am making a cake with a starburst for my husband's 70th birthday and I would like to include trailing and standing streamers. Any ideas please?
Without a clear picture in my head of what you are planning, I can't say for sure, besides this:
You will simply support the streamer or ribbon in the shape you want it as it's drying, but horizontally instead of vertically. Use plastic wrap, tin foil, egg cartons, etc, to add peaks and valleys to your ribbons as they dry. Maybe even add a wire the whole length through, depending on how long you want these things to be.
Use gum paste or fondant with tylose powder. You could also use modeling chocolate. Again, without seeing exactly what you have in mind, it's hard to give specific help.
Make extra, breaking will occur.
Don't move the pieces until they are COMPLETELY dry.
Post photos when done!