You've been soaking the cubes so you know how much they expand. So use whatever container, glass is best, you've been using and cover the cubes with the same amount of water you've been using to soften the cubes. The only difference will be you'll add about two teaspoons of yogurt. You can mix it into the water or not. Doesn't matter. The active culture will multiply and fermenting will start.
All fermenting does best when kept at a warm room temperature of not below 70F. At 75F, it will ferment faster. The first batch could take 36 hours to 48 hours. You'll know it's fermented by the tangy odor. If it smells putrid, toss it out. The smell should be pleasant, not rotten.
You can add more cubes to the "juice" or save a tablespoon of juice and use it to start a new batch with fresh water. The second batch should ferment in as little as 24 hours.
