I've never done this before, but here's my suggestion: Cut a wide ribbon (maybe a wired-edge type) to the height of your cake, including a drape or swag length. You can then hot glue your leaves to it, overlapping them and positioning them. Then you can simply attach the ribbon to the top and "drape" your ribbon of leaves down the sides.
Try sticking them on with a little buttercream or royal icing.
Off the subject, but that is a beautiful cake!
If the cake is fondant, you can use melted chocolate or RI.. If the cake is buttercream, you can use buttercream..
Have you thought about what dyes are in those leaves; that someone will be eating the cake where they touched the icing? Some of those colors bleed into the icing
You can make frosting sheet or wafer paper leaves that would be food safe.
Using a trail of rib bon is probably the best suggestion. Glue them w/melted candy or royal icing or the hot glue as suggested.
The ribbon idea is good, I just don't want it to show up but I'll pick something similar to the colors so it can be less noticeable.
Kakeladi... I though about the dies... the other day I put some candles on a cake for home and it had glitter. It was all over the fondant and they were there for a few seconds while we sang Happy Birthday so I really have to be careful with those things.
You can use either gum paste or fondant. Have in mind that gum paste dries out faster. You'll need the autumn leave cutter. Knead the past (either one is good) in an orange color. Cut the leaves and paint sparingly in brown and green duster lust using a brush then crunch some aluminum foil and throw the leaves on it to allow them to dry out. As they dry they will be taking the shape of the position they got when you threw them over. For a final touch you can brush some pearl dust for the shine.