Fill a plastic bucket with water, stopping 1 inch from the top. For colored ice candles, add as much food coloring to the water as you like. A few drops will make a light-colored candle; a spoonful adds a deeper hue.
Fold up a blanket to insulate the bottom of the bucket. Set the bucket outdoors on top of the blanket.
Lay the yardstick flat across the top of the yogurt container. Tape the container to the yardstick by running a piece of duct tape across the yardstick, down one side of the container, across the bottom, and up the other side. Tear the tape and overlap the pieces on top slightly to secure them.
Lay the yardstick across the bucket so the yogurt container is in the center of the water. This will form the hole into which you place the candle.
Allow the ice to freeze overnight or until solid.
Fill the yogurt container with hot water until the ice around it melts enough to remove it. Remove the container and yardstick, leaving a hole in the ice.
Turn the bucket over to remove the ice. If necessary, pour a bit of hot water over the bucket to loosen the ice on the sides of the bucket.
Turn the block of ice right-side up so the hole from the yogurt container is on top. Place a tea light or pillar candle in the hole; light the candle with a long match or lighter; and watch the ice glow.