I have no idea which solvent is the best for recrystallization of mephedrone but the substance doesn't need to be iced first.
The principle is: pick a solvent in which the product is soluble when hot but insoluble when cold. The impurities must be soluble at all temperatures (or at least more soluble than the pure product).
If the ideal solvent were absolute alcohol, you'd put your substance in a flask, then heat the alcohol in a beaker till it's practically boiling. You drop the hot alcohol onto the solvent with a pipette (medicine dropper), being very careful not to add too much solvent. That's usually the most important step: only adding enough hot solvent to dissolve the substance. If you add too much, you have to distill and start over again.
Also extremely important is that all of the substance goes into solution: if it's cloudy or milky, the procedure is pointless.
Then after that, yes, just let it cool on its own. If room temp is reached and you don't have crystals, ice it and and scratch the inner walls of the flask with a glass stirrer. If you have some pure mephedrone, a grain or two can be used to seed the crystallization (like clouds are seeded by nucleation centers) if necessary.
Then gravity or suction filter.
There are variants to this technique--like hot filtration. If you had more info on purification of mephedrone, it'd be very helpful.