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.

A teaspoon is too much. Most recipes I have seen make about 750g ice cream base. That's 0.66%, which is definitely in the thickening range. Try adding less than 0.1%, so 0.5 to 0.7 g guar gum for a 750g recipe. If you don't have a precision scale (they are available for about 10 Euros online, so if you use guar gum, dry yeast and similar ingredients often, it is a good investment), make it a small pinch, or the tip of a knife, but be prepared to have large measuring errors.

Against the clumping, just don't throw it into the mixture like that. Gums clump immediately when they touch moisture, much stronger than starch. It is best to first mix thoroughly the powdered gum into a tablespoon of your sugar, then mix this sugar into the whole batch of sugar, and then use the sugar as usual. If you decide to add gum after the mixture is ready, you should do it in the mixer. Put the mixer at a medium speed, put the gum into a spoon, and slowly tip the spoon into the wind caused by the beaters, so that the powder reaches the mass as a flurry of separate particles and is immediately dispersed by the beaters.

Changing to xanthan gum won't help much, as both have the same clumping problems and have to be used in similar concentrations. If you have an easier source for guar, it is not worth looking for xanthan, unless you want to use both at once for the synergy effect. But this gets interesting when you want to thicken with them, you don't need this effect in ice cream.

Locust Bean Gum can also be used for the same purpose.
These vegetable gums are classed as "stabilisers". Basically, the fat molecules and the water were blended at the (hot) making, but they want to migrate apart. They do so very slowly in the freezer.

Stabilisers such as Guar Gum, and Locust Bean Gum, and Xanthum Powder serve to reduce the speed of that migration. The effect is dramatic.
3 years ago, when starting our own ice cream production business, I wanted to see if we really needed to add the gums. I made a batch with, and a batch without the gums, and left them in the freezer for a month. We then tasted both. The one with gums seemed as good as the day after it was made, terrific. The one without gums had gone icy through and through.

The gums are best mixed with sugar, and dry-mixed at that point very well.
Then add this mix to the still-cold milk mix. Do it slowly, or you will get lots of big lumps. Add when cold, as this helps as well, because the gums become more activated in their swelling as the mix warms. Be sure to blend very thoroughly once you have added the gums, and before heating the mix.

I think you're on the right track with the guar gum. It should help with the shelf life, and reduce large ice crystals too. The trick is to add it slowly while spinning the mix in a blender. That keeps it from clumping. Try adding a fourth of a teaspoon at a time, checking to see how thick it's getting. I know from using xanthan gum that you can see an immediate difference each time you stop the machine. Just get it thick enough that you can barely start to see a difference -- if you add too much, it'll start having a real sticky, gloppy, texture, and be somewhat chewy. I don't think it'll take much to make a difference with the problems you're having.

I made a raspberry sorbet once a few years ago with a little guar gum and I believe I just took a sharp knife point and and whatever I was able to pile on to the tip (up to about 1/8" from the tip) and added that to my mixture.

It turned out great; even that small amount made it a tiny bit "slimy" but was a great ice crystal inhibitor. I think a tiny bit goes a long way: 1 tsp. or even 1/2 tsp. would be way too much for my own 1 1/2 qt. capacity ice cream/sorbet maker.