The fireball is by far my favorite offensive attack in LDs. It takes a little practice, yes, but there are times where I just don't have the energy to do it for whatever reason. But anyway, the way I do it is to draw energy from my solar plexus through my arms, into my hands and then between my hands. I try as much as possible to feel the heat from it. Sometimes, chanting a spell or mantra can help a lot. Then I throw it and will it to explode on contact with the target.