If you don't have much experience with cake decorating, getting nuts to stick to the side of your cake may seem like an accident waiting to happen. Rather than attempting to tilt your cake and sprinkle the pieces on it, use a simple pressing technique.
Frost both the top and sides of your cake with a flat spatula. Apply an especially thick coat of frosting around the sides of the cake so there is room to press in the nuts without them poking into the cake.
If your frosting is very firm or your cake has been refrigerated, wait until the frosting reaches room temperature. Then moisten the frosting with a flat spatula that you've dipped in water. Soft frosting holds nuts better than firm, hardened frosting.
Finely chop the nuts you intend to use. Whole nuts are harder to press into frosting and less likely to stick well.
Set the cake onto a flat plate and lift the plate up over a shallow bowl or baking sheet that's wider than the plate. With your free hand, scoop up a large handful of nuts. Using just a few at a time means you're more likely to smudge the frosting, so pick up more than you need and reuse any that fall.
Gently press your handful of nuts against the side of the cake. Some nuts will likely fall into the bowl or on the baking sheet below; simply collect them to use again. Rotate the cake as you work so you end up with an even coating of nuts all the way around.
If you end up with "bald spots" on your cake after the first pass, moisten those areas slightly and then press more nuts into the frosting until you achieve an even coating.
To embellish the decorating, you can press whole nuts in a pattern on top of the cake, or press more finely chopped nuts into an even layer on top.
When you finish decorating, chill the cake for at least 20 minutes after pressing wax paper against its sides to further secure the chopped nuts.