Prepare the molds. Wash and pat them dry and set aside.
Prepare the fruits if using. For the mangoes, peel the skin, and cut the flesh on both sides of the seed. Slice each side into thin stripes. Roll the strips to form roses and place them on a freezer safe tray. Use toothpicks to hold the rolls intact in necessary. Once the rolls are completed, place them in the freezer to set for about 30 to 60 minutes. Freezing makes it is easier to handle the rolled mango strips.
For the oranges, peel the skin and divide each fruit into 3 to 5 segments. Slice each segment thin and place the slices into the prepared jelly molds. Set aside.
Combine the konnyaku jelly powder and sugar in a small bowl. Mix until both are well combined (see Note 1). Set aside.
Add water into a medium sized pot. Turn on the heat to low. Once the water is warmed up (should not be boiling), add the sugar-jelly powder mixture into the pot. Stir constantly as you add the mixture into the water to avoid clumping.
Cook the solution on low heat (stirring constantly) until the sugar and jelly are all dissolved. This should take approximately 5 to 8 minutes over medium low heat.
You can check if the jelly is cooked by scooping some of the jelly solution into a spoon. Wait until the bubbles subside and if the solution appears clear, the jelly is cooked. If it appears cloudy, you will need to cook it longer. There will be tiny bubbles in the jelly solution as you scoop it out in a spoon, so hold it for about 20 seconds to allow the bubbles to subside. The bubbles can make your jelly look cloudy (see Note 2).
Turn off the heat (see Note 3).
Towards the last 2 to 3 minutes of the cooking time of the jelly, remove the mangoes from the freezer. Remove the toothpicks if you have used them, and start to arrange the fruits into the jelly molds.
Once the jelly solution is cooked, spoon it into the molds. You will need to work fairly quickly as the jelly tends to set as it cools down.
Let the jellies rest at room temperature for a few minutes before transferring them to the refrigerator to set completely (for about 2 hours) before serving (see Note 4).
To unmold, turn the mold over and gently press the top or tap them on a wide plate to release. If they are stuck, gently loosen the sides by pushing the sides away with your fingers before turning the mold over and tapping them out.
Keep the konnyaku jelly refrigerated until ready to be served.