As much as I like to keep things simple, I do like doing things big, so get a good-sized enamel pot for this.
Fill with 8-12 cups of water.
Then, add your herbal simmer ingredients!
I tend to not measure, but I’d add something like 2 lemons sliced and 8 or so sprigs of fresh thyme (or about 1TB of dried). Or a handful of dried orange peels, a bunch of thyme sprigs and some oregano.
I’m really into lemongrass these days, so I’m likely to make a lemongrass and lime-leaf simmer soon.
If you want something more complex, feel free to add whatever your heart desires!
Once all of your ingredients are in the pot, bring to a boil, let boil for a minute or so, then reduce the heat to a low simmer. Because you are using a huge amount of water, you can walk away for a couple of hours, check on everything (add water if the water level is below half what you started with) then go about your business for another couple of hours.
After four hours, turn the pot off, let cool, then discard the water and herbs.
Note: You don’t have to use big amounts of herbs and water like I do, you can use 4 cups of water and add water every half hour or so to keep the liquid from getting too low.
If you want to keep an eye on the ambient humidity in your home, I recommend getting a hygrometer. I use this one in my violin case and feel it is accurate enough for my not-a-fancy-Stradivarius violin humidity needs: https://www.amazon.com/gp/product/B00JXOKQVW?ie=UTF8&tag=quaternityhol-20&camp=1789&linkCode=xm2&creativeASIN=B00JXOKQVW (affiliate link) Place the hygrometer in your bedroom.