Final answer:
Creating icons on SketchPad in Sims 4 cannot be done directly in-game because the functionality does not exist; however, players can design icons outside of the game using graphic software and import them as custom content using tools like Sims 4 Studio.
Step-by-step explanation:
To create icons on SketchPad in Sims 4, you'll need to use the in-game digital drawing tablet, which your Sims can purchase and use to enhance their Painting skill. Unfortunately, creating custom icons directly within Sims 4 using SketchPad is not possible because the game does not offer this feature as part of its functionality. Instead, players can use external graphic design software to create icons and then import these custom graphics into the game through modding or creating custom content (CC). To make custom content available in the game, you would have to follow steps such as creating the artwork using a graphics editing program, saving it in a format compatible with Sims 4, and then using Sims 4 Studio or a similar tool to import the creation into the game.