This is the most common and easiest way for a crystal to grow.
The crystal will begin to grow only when the solution is saturated. Let me define “saturated“ for you.
In this case, saturated solution is when we cannot dissolve anymore crystal growing chemical in a certain amount of water. This is better called balanced solution.
At this point crystals should be able to start growing, as long as there is a seed or a little disturbance in the solution such as motion or a dust particle to begin the process.
It is also possible that nothing grows at this point, this can happen when nothing mentioned on the last paragraph is present. In this case, we just have to wait until the solution turns super saturated or enough water is evaporated so the solution becomes unbalanced and is forced to pop out at least one molecule of chemical out of a molecule of water and begin the growing process.
As water evaporates crystals will grow and you can control how fast water evaporates by covering the vessel totally or partially if you want slow evaporation, or in the other hand you can use a fan to accelerate the evaporation if you wish.
So, slow evaporation delivers clear and big crystals and fast evaporation delivers small and foggy crystals.