Simply put; you heat metal it gets malleable. If it get hot enough, a.k.a. the melting point (science), solid forms transform from solid to liquid state where in the the can be cast to a form. While I usually encourage trying things at home for yourself, this process should be performed with great caution in the proper facilities. However if that scenario is accessible to you I highly recommend you give aluminum casting a try.
That being said aluminum casting is a process that involves just that, applying heat to aluminum in order to form it. A stone furnace is outfitted with a gas line torch used to apply heat to a bowl loaded with solid metal. The bowl is known as a crucible. It is usually made from charcoal or ceramics. These materials having a much higher melting point than metals especially aluminum (1,221ºF). Once the metal in the crucible reaches liquid state, the crucible is removed from the stone furnace. You will need two people to achieve this operating crucible tongs. The glowing red crucible is then place on either a sand base of stones. At this point the slag or impure metals are scraped off the to of the liquid. The two people will then work in tandem to lift and pour the liquid metal into a form.
A great inexpensive material used to set liquid metal is sand. Sand acts as a great material for the liquid metal to cool, due to its property of being very effective in dispersing heat. The fact that it is very fine in its grain allows for detailed impressions to be pressed into wet sand and act as a negative mold for the metal to fill in. Sand will repel the metal. Once poured let the aluminum sit for 20-30 minutes until its is cool to touch. At this point it can be removed from the sand and rinsed off, leaving you with a detailed positive of the mold.