I experimented with it and wet from the consistency of cooking oil to that of hair conditioner.
All I can really suggest is figure out the number of cups of water you want to use. Then measure out one teaspoon for each cup of water. Example: If you want to use 1 gallon of water, that's 16 cups, so you'd need 16 teaspoons. Add, and mix vigorously (use an electric mixer. You can't to it by hand).
Rinse and repeat as necessary until you get the desired consistency.
If metric is more your style, 1 gallon is roughly 3.75 liters, 1 cup is roughly 250 gram/milliliter, 1 teaspoon is roughly 1 gram. There's a lot of wiggle room in those conversions.
Here is a really useful video that shows the viscosity of xanthan gum solutions from .25% to 6%. Personally I think 2 to 3% is the sweet spot, but depends what you're looking for.
I used a 1% xanthan to water mixture e.g. 8 liters of water (8000g) to 80 g of xanthan gum. 1 liter of water weighs 1000 g so multiply by 0.01 to get the amount of pwder. I use a digital food scale and add the xanthan to a mixing bowl and weigh it as I go. Once I have my measure of xanthan I mix a generous amount of cooking oil to the bowl and stir it to make a paste similar to molasses. Generally just add a puddle of oil stir, if it is clumpy add more oil and stir. Then I fill up a bucket with hot water from the tap, I don't boil it then add any colouring to the water. I give the gum/oil paste a quick stir just before adding to the water so the consistency is even. Then I add to the water and mix with a hand held blender for about a minute give or take. I just look to see how consistent the mix looks and dip a finger in. The gunge will be ready to use right then but it may be hot. As it cools down it will get thicker.
Main points: -10 g of xanthan powder to 1 Liter of water -add oil to xanthan first and stir to a gloopy paste in a mixing bowl -Add hot water to bucket and colouring and stir until the colour is consistent -add xanthan paste and mix with electric blender for ~60 - 90 seconds.