[yiyo]

sorry for the late reply.
if i use two solvents in practice, then i should use one of the solvent that sample dissolves more in both hot and cold conditions. and the sample doesn't dissolve the other solvent in both hot and cold conditions....?

[Babcock_Hall]

The way that it is most commonly performed is that one dissolves the solute into a small volume of the hot solvent in which it is more soluble.  Then one adds the second solvent dropwise until cloudiness is seen and persists (still while hot).  Then one might add a small amount of the first solvent to dissolve the cloudiness, and then one cools the solution slowly.