Simply prepare a supersaturated solution by dissolving solute into heated solvent until undissolved material starts to accumulate. For example, you could stir borax into hot water until it stops dissolving. Allow the solution to cool and pour off the supersaturated solution. Seed the supersaturated solution with a few crystals or grains of solute and seal the container. Twice daily, shake the container. The excess solute will accumulate on your seeds. After 2-3 days, you'll have a saturated solution. Filter out the undissolved material and store the saturated solution or use it to grow crystals.
Three Ways To Make a Saturated Solution | Crystal Growing Recipes