Never tried this myself but here's from an article I found on a quick search-

How to Take Borax Internally

Earth Clinic's Ted recommends that men take 1/4 teaspoon (940 ~ 1,000 mg) of borax dissolved in one liter of water. This liter of water is then sipped throughout the day.

Ted recommends 1/8 teaspoon (470 mg ~ 500 mg) of borax dissolved in a liter of water, sipped throughout the day, for women. This borax water is taken 5 days on and 2 days off. For example, take the Borax water Monday through Friday and take the weekend off.

Some people add the borax to their morning coffee or sprinkle it on their food as they would salt. It would be wise to measure out your daily dose (1/4 teaspoon for men (940 ~ 1,000 mg), 1/8 teaspoon (470 mg ~ 500 mg) for women) each morning if you choose this method to make sure you are not taking too much.

While borax is no more toxic to the body than table salt, taking too much may cause temporary stomach upset or diarrhea. Even the regular dose may be too much for some. If you find that the ¼ or 1/8 teaspoon daily does not agree with you, cut the dose in half. Or you can use the much more diluted version recommended by Walter Last, which many have found to work quite well for their needs.

Walter Last recommends dissolving 1 teaspoon of borax in 1 liter of water and then taking one or 2 teaspoons of the borax solution daily.

https://www.earthclinic.com/remedies/borax.html

Read this and they provide info on how to make it. It's as simple as adding sugar to a cup of tea really.

Personally, I simply at a rounded teaspoon to some heated water. I use reverse osmosis water. Stir it until the borax salt dissolves. You don't need to heat it up much for the borax salt to completely dissolve. Use a quart jug or liter container (I use a glass container) and pour the dissolved Borax into the quart/liter container. Add RO water to get a liter or quart amount. Then keep it in the frig.

I use a medicine cup and measure 35 to 40 ml of it every other day. The article I provided you tells the newbie to start small and work up to taking more. I mix it with fruit juice, but if I'm in a rush I will pour it in a glass of water and drink it. If you read you'll find out very quickly from all those censored studies, that arthritis is largely preventable and reversible. Boron manages calcium levels other wise it builds up in arteries, the brain, muscles, and bones. We get too much calcium. Ever wonder why there's such a push on calcium? Some one said -- the amount od calcium we're getting is turning us into coral reefs.