I read about others who have trouble filling it and the only thing I can assume is that they don't close the lid so it fits properly. The top part has a rubber ring that must fit correctly into the bottom part to prevent leakage. Seems simple to me. Personally, I've really enjoyed it. One of life's small pleasures, making round ice cubes :D

It is extremely difficult to fill and retain water inside in order to release it when it freezes into a spherical shape

A poorly designed product because simply the rubber top can only stay attached to the base with tricks when filling with water. Simply because the "attachment" is not stable enough to withstand the water level that rises inside. Eventually, after some failed attempts, I managed. 1) you attach the two halves 2) you pour water on the top externally to gain weight 3) you carefully fill the holes with water using the funnel until they overflow 4) you carefully transfer the ice trays to the freezer because here is the great danger of the two halves coming undone and the water emptying. Also, when the ice cubes are formed, this rubber (we call it silicone!) sticks forcefully to the ice cubes and it takes quite an effort to detach the two halves! It is a process that takes a lot of time but it is worth it because these ice cubes do not melt easily, which is sometimes desired.

(I tried it twice, it doesn't come out spherical at all)

I finally managed to do it. We fill the base, put the top part, fill it with the funnel, until it overflows enough for each ice cube, and 4 beautiful spheres come out.