Prepare a nearly saturated solution of your compound at or close to the boiling point of the solvent of your choice. Transfer the solution into a clean container and cover. Place the container into a heat bath at about the same temperature and allow to cool slowly. A Dewar with hot water frequently does the trick.
A variation of this method is to prepare a saturated solution at room temperature and place the container in a cold place. E.g. thf is still liquid at -80°C, which allows growing crystals in a dry-ice acetone bath (or in the -80°C freezer).
Advantages: Easy, works best for ony moderatley soluble substances.
Disadavantages: Needs a lot of material, starts with saturated solution (too many small crystals), usually takes place at high temperature, which can lead to disordered or twinned crystals.