Are you fermenting it to the point where it starts separating? Are you doing a second ferment without the grains?

If you’re looking for a smoother/less vinegary taste taking the grains out a bit before the separation stage is ideal— it will have started to thicken but not separated. Sometimes you need to mix it to see that it’s thickening.

A second ferment in the fridge helps mellow the flavor out significantly. Put the freshly strained batch of kefir in the fridge with a tiny piece of fruit and leave it for 2-3 days. Raspberry works well, nutmeg also makes a good flavoring on its own.

Fermenting both stages with a lid will make the flavor more mellow than using cheese cloth or other more ventilated options.

Generally, fermenting quickly will be more tangy and fermenting more slowly will be smoother, so fermenting at higher room temps can effect the flavor.

To help address the nervousness I recommend tasting it during the process— taste an hour after beginning fermenting then 3 hours then 12, then taste after each day of the second ferment. I think day 3 is perfect. Smelling like yeast starter is probably good.

If it smells like rotten milk (which will be obvious) than it’s bad and you should rinse the grains with milk than put them in a smaller amount next time so they’re able to colonize before the milk spoils. A teaspoon of grains can usually handle about 12oz milk (for a 12 hr ferment), but they can take time to adapt to eating a new milk brand.