You need to place decorations next to the allocated house/business. If you hover your mouse over the building you'll see what % you are at, and if you look in the Shop under the Decorations tab, you can see what % each decoration is by also hovering your mouse over it. Ensure you have a close look as some decorations boost gold output while others boost your goods output – this is used when needing to boost a farm plot (See Article 'Boost Farm Plots').
Once you have purchased a decoration and you place it near a house/business you will see its radius – ensure the building you are trying to boost is within this.
You can also see the radius if you hover your mouse over the decoration before purchasing it.