To do plain yogurt you use a yogurt "machine" for few dollars like this:
Then you mix UHT milk (it has to be UHT to be steril, otherwise yogurt bacteria cannot grow, they are weak ...).
You add a few spoons of plain yogurt, which has its own bacteria alive, switch it on and during 8-9 hours (by night for example) you get it.
Pour away the liquid you'll find above and keep the white cream: that's your no sugar yogurt (only milk carbs).
Put it in the refrigerator and it should last 3-4 days.
The only "difficulty" is to find out the right time to leave the yogurt building up: too few and it's liquid, too much and it gets too acid.
Try and repeat ;-)