If you wish, you can light some candles and incense, or play some soft, calming music while you bathe. The mood should be one of calmness and purification. Any scents chosen should be gentle and light.
Once you're in the bath, systematically relax, taking a minute to tense first the balls of your feet, then your calves, then your thighs, working all the way up your body until you've given each muscle group a tensing and relaxing. As this happens, imagine your stresses, worries, and concerns being gently pulled out of your body and absorbed by the salt in the water. Most tubs aren't large enough to lay fully down in, but make sure you get your whole body covered with water. Splash or pour some on your face, shoulders, neck, ears and other places that can't get fully immersed in the water. I usually flip over to my belly for a few minutes to give my chest some time immersed.
No soap is to be used in a spiritual bath. Make sure that you're physically clean before entering. If I need, I'll take a quick shower first before having my spiritual bath. The spiritual bath is meant specifically to clean the aura, not the skin.
You can do spiritual baths as often as you like, whenever you need, but I recommend doing at least one a week. I typically choose the end of the day on Friday as a good time to wash off the stresses of the week and renew myself before the weekend. Keeping good spiritual hygiene means that fewer unwanted energies will be able to attach themselves to me in general. Spiritual baths are also good to do in emergencies. If a situation you're in is really starting to stress you out, one of the best things you can do is start with a purifying, cleansing bath.
Feel free to experiment with bath recipes to find one that really works nicely for you. Some people like to use epsom salts as well in their baths. I always add a cup of apple cider vinegar to mine as it seems to add more purification power to the water. Milk is a wonderful addition that is great for your skin. You can also brew an herbal infusion and add that to the bath if you want to bring the essences of those herbs into your aura. Do not add herbs directly to the bathwater unless you want clogged drains and a messy tub to clean up afterward.
Keeping good spiritual hygiene has been an important part of my practice for a long time, and it has served me well. The spiritual bath is a wonderful tool that anyone can use anytime to refresh and renew oneself and to remove unwanted influences.