Like many installments in the series, customization is a key feature in Animal Crossing: New Horizons, and there are multiple ways to have your own designs in the game; either creating new ones or importing some old designs.
To personalize furniture you must first participate in a small section in the workshop Tom nook at the Resident Service until you get a personalization kit to use at home. With this you can modify most of your objects and tools, including many of them bought in stores.
You will only have to access a workbench and select the option to customize, although you will need a certain number of customization kits per object to perform the redesign. Another option to access personalization kits is to buy them in the store of Nook
It is also possible to import the designs of the popular Animal Crossing: New Leaf Y Animal Crossing: Happy Home Designer from Nintendo 3DS. For this you can use the new service Nooklink through the app Nintendo Switch Online, which will allow you to scan QR codes of a design 3DS
Remember to keep your designs simple in principle so that they do not end up very blurred and can be understood. You will not be able to use the Nintendo Switch for your designs, but yes to create news in the table of news of the town.