The crumb coat
Applying a 'crumb coat' is the best way to achieve professional-looking results, especially if you want a smooth-sided cake. This is essentially a thin layer of icing that will help seal in the crumbs so that your final coat of icing will be that much easier to apply and crumb-free.
Have handy your cake circles, sugar syrup, icing and a damp sponge or cloth for wiping away stray crumbs.
Set your least-perfect cake layer on a cake circle, and apply simple syrup with a pastry brush. Add a healthy dollop of icing, and spread it across the surface with a palette knife, almost to the edge - the weight of additional layers will spread it out further. Top with another layer of cake; if you have a cracked or broken layer, sandwich it in here. As you add layers, bend down so the cake is at eye level: ensure the layers are stacked flush. Apply sugar syrup and icing at each step. When you reach your final layer, apply syrup and icing, spreading it to the edge of the cake. Add a dollop of icing to the side of the cake, and spread it thinly to cover as much as you can.