Most sulfates are insoluble except for Group 1A elements. Hence, lithium sulfate is soluble in water. In order to make a supersaturated solution, add solid lithium sulfate in any amount of water. Then, stir. Don't stop adding until some of it will be left undissolved. Once particles settle and doesn't dissolve no matter how much you stir, heat the solution while adding more of the lithium sulfate. When it dissolves all of the solute, you have now created a supersaturated solution.