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.
See the pictures below for clarification:
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