Simply put, you dissolve a sample of solid potassium iodide in water.
Since you didn't give any detail about your target solution, I'll show you how to make an unsaturated, a saturated, and a supersaturated solution of potassium iodide.
Here's how potassium iodide's solubility graph looks like
So, let's say that you want to make an unsaturated potassium iodide solution at
This means that, in order to make an unsaturated solution, you need to dissolve less than 160 g of potassium iodide per 100 mL of water.
To make a saturated solution, you need to add enough potassium iodide per 100 mL of water to match the solubility at that respective temperature.
In this case, if you slowly dissolve 160 g of potassium iodide per 100 mL of water at
To get a supersaturated solution, you need to find a way to increase the amount of solvent that can be dissolved per 100 mL in your solution.
Let's say that you want your solution to dissolve as much as 180 g of potassium iodide per 100 mL of awter at
To do that, you heat the saturated solution to, say,
After you dissolve 180 g of potassium iodide in the solution, you slowly cool the solution down to
If you manage to do this correctly, the solution will now hold 180 g of potassium iodide per 100 mL of water at