Considerably more difficult than forward crossovers, backwards crossovers take a great deal of practice to perfect. The main typical problem, is maintaining balance while on either foot and travelling backwards.
Various slightly modified techniques are used, all with slightly different effects. As backwards crossovers are learnt it will become evident which is your preferred method.
The original backwards crossover method, is the exact reverse of forward crossovers. Skates are stepped one by one over the other as the skater turns. A typical alteration to this method involves keeping parallel, but turning using the front edges of the blades.