How can crystals of zinc sulphate be formed from a dilute solution of zinc sulphate?
The solution could be allowed to evaporate; and while water evaporates slowly, this is feasible.
Alternatively, we could layer the solution with a solvent in which ionic solutes are (relatively!) insoluble. Ethanol, which is miscible with water, is a good bet; many ionic solutes precipitate upon addition of ethanol. If you want macroscopic crystals, you could enclose a beaker of ethanol, and the beaker of zinc sulfate solution under a bell jar, and the ethanol would slowly diffuse to the zinc sulfate solution, hopefully crystallizing the zinc sulfate slowly.
Please note that you have to work at making crystals. Inorganic chemists, who have to grow good, regular crystals for X-ray diffraction experiments, KNOW this frustration very well.