How do I add guar gum when making ice cream?
Where are people getting the idea that Xanthan gum comes from corn?It does not.  It is harvested from bacteria (Xanthomonas campestris).  

If you are using a cook-up custard to start with, your custard will thicken but will also get ropy or the term is snotty when you pour a spoonful out as you stir.  That is definitely a good way to tell if you are using too much Xanthan gum so you need to cut your amount down.    You sometimes see that effect in cheap, fat free salad dressings.  

It takes less gum than a starch because a gum holds much, much more water than a starch. Where you might use a tablespoonful of cornstarch, you would only use maybe 1/8 teaspoon of gum and that might be a little too much. Gums go into liquid if they are mixed with other dry ingredients or they have heat treated to dissolve instantly.  Those are usually only sold commercially so you would not be able to buy them on the internet.    

My husband makes a "to die for" cooked vanilla and chocolate custard and freezes it.  But it is as hard as a rock after freezing.  I am still working on him to let me modify the formula to give us softer (not soft-serve) ice cream.  I have a masters in food science but I am going to have to make a separate batch to prove it to him.  To most people, anything other than flour or corn starch to thicken must be a chemical.  Hope that helps.