Follow the spells instruction on when to do the spell or pick a moon phase that corresponds to the spells intent.
The simplest way to cleanse both body and energy is a bath with a tsp of sea salt added. If desired, add corresponding herbs and/or oils to the bath. You can also burn candles and/or incense if you like.
Afterwards, put on clean and plain clothing and any magical jewelry you like. Correspond color of clothing with spell intent if you like.