Start by creating and formatting the very simple viz you want to appear in the tooltip. Simple is better in this case, and it should be as unfiltered as possible to start.
On the sheet you want to have a viz tooltip: Doubleclick the tooltip marks card to open up the edit menu. Here you can customize the tooltip text, including referencing the sheet name of the viz you made for your tooltip.
You can also do this with Insert > Sheets and select your sheet.
Step 3: Profit!
Now when you place your sheets in a dashboard, your viz will appear on hover.
In addition to Dashboards, Tableau also has stories, which allow you to guide your reader through sheets individually. Your sheets here are formatted exactly as they appear on their sheet.