I`m new and need small help

Whatever you pour the silicone over will be the shape you get from the mold, the shape will be set and there will be no changing it. To get a specific shape, you have to start with that shape and pour the silicone over it to get that shape from the mold. And no, you don't have to heat the silicone, you only heat it if you want it to cure a little faster or if you have a small mold that needs help with small details. I hope this helps, feel free to ask if you need more help.