There are many ways to form crystals. Some ways are described here:

Evaporation: Crystals formed by evaporation form due to the fact that there is decreasingly less solvent available for the ions to be dissolved in, therefore causing a state of metastable or supersaturation in which nucleation and crystal growth can occur.

Adjust the pH: This method is commonly used to obtain homogenous nucleation and growth because we can control pH by various reactions. In doing this, we can effectly make the change as slow as we want and make it completely uniform throughout the solution. The crystals again form because the extent to which the ions are soluble decreases when the pH is changed in the right direction.

Lower the temperature: By lowering the temperature of the solvent, you can reduce the solubility of the ions. This is better explained here.

Change the nature of the solvent: This method is commonly used when a faster crystallization is desired. In order to change the nature of the solvent, one would normally add non-solvent (something which the ions are not soluble in, or not as soluble in) to the current solution. By doing this, you can effectively reduce the solubility of the ions in the solution and cause rapid precipitation . There is an optimum amount of "non-solvent" to add to the system. Usually there will be a slight degree of solubility in your "non-solvent" and there adding too much with allow the ions to stay in solution dissolved in your "non-solvent".