Hi ruwaida5
Thanks for posting the cake again - its beautiful!
To ensure that the ribbons are even around the cake is - before you begin, make sure that the sides are the cake are VERTICAL from the bottom upwards! Note how NEAT and STRAIGHT the sides are on your SAMPLE cake.
When you have the first ribbon on, measure the exact distance above this one - where you want to position the second one - marking the fondant around the sides of the cake at intervals, with a cake scriber or an edible ink pen. This should help to keep the line space between the ribbons STRAIGHT.
Repeat this step with the 3rd and 4th ribbon - ensure all joins are facing in the one direction at the back of the cake
The other option would be to use a wider black ribbon, and position the narrow white one over the the black one - to give a similar effect.
Good luck with the challenge!
Hope this helps!
Margaret393