Iam not a expert at this but I am trying my best.  OK so I'm making a smash cake for my sister.  I start decorating with #23, top is fine now I get to the sides and the icing just basically drooped out of its shape and ran down the cake.  This happened 3 times.  What am I doing wrong? 

It could be a couple of things. One is the icing is too thin, if it is to thin it would be okay on the top of the cake where gravity doesn't affect as much. If icing is too thin it won't stick to the side due to gravity. A second cause could be if the icing is butter based the heat from your could be making to soft. 

I agree -it sounds like your icing it much too thin.  You can thicken it up by adding some cornstarch and/or powdered sugar.