At this point I am washing my 2 Sphynx Cats once a week with water but I look forward to see your routines and what products (if any) do you use for yours.
Do you even need to wash your cat?
Usually, cats wash themselves by licking their fur and don't need to be bathed by humans. But with Shpynx cats, the situation is a different one.
Skin always produces natural skin oil, called "sebum". The hairs of cats would usually wick most of that sebum away from the skin, where it is then licked away by the cat. Since Sphynx cats don't have fur (in the common sense of the word), the sebum stays on the skin and makes it oily over time.
So yes, Sphynx cats need to be washed to reduce the oil on their skin. The same goes for ear wax. Since Sphynx cats don't have (much) hair in their ears, you need to clean them regularly.
That greatly depends on your cat. I've seen several sites promoting a weekly bath, but personally I would only bathe them as needed.
Just like some people have dry or oily skin, so do cats. If your cats have dry skin, a weekly bath could dry them out even more and lead to skin irritation and infection. If they have oily skin or like to wallow in dirt, they might require more frequent baths. And the more you bathe oily skin, the more oil it produces, creating a spiral of ever shorter times between baths and ever oilier skin.
Here's a forum with people sharing their bath schedules. Most bathe their cats every 10 - 14 days, some weekly and very few even more often.
How to bathe them?
Please use shampoo specifically formulated for cats, which you can buy in many pet stores or online. Human shampoo is too acidic, can irritate and dry out the skin and it shouldn't be ingested (if the cats lick themselves during the bath). Please read more in this question and this question.
Never use essential oils on cats, most of them are extremely poisonous to cats.
If the cat isn't particularly dirty, just plain water can also suffice. If the skin isn't dirty or oily, the cat doesn't need to be washed at all.
You can either bathe them in mildly warm water or wash them with a wet cloth. All residue of shampoo should be completely rinsed off. They should be dried with a towel and kept warm until the skin is completely dry.
To clean the ears, please don't use q-tips, the risk of injury is too high. Instead wet a cotton pad or some kitchen towel, squeeze out all excess water and gently wipe the inside of the ear clean.
If the cat has very oily skin and isn't washed regularly, it can get feline acne. However, if you notice dark spots on your cats chin, mouth or nose, you should first discard any food and water bowls made of plastic. Mild food and plastic allergies are very common in cats and often cause blackheads or brown spots. We have an example in this question, but blackheads and itching at the chin are also common.
Please use glass, porcelain or stainless steal bowl instead.