This is provided for informational purposes. Melting metal is always dangerous. You are responsible for your own safety if you decide to try anything described here. tl;dr Don’t try to sue me because you did something something stupid.
Melting aluminum is not particularly hard or expensive to get a basic setup. You just need to know how to get the metal hot enough to melt.
In the past my setup was a 2-3 foot across pit in the ground with an old rusted out dutch oven to hold the molten metal and a hair dryer attached to a metal pipe with a PVC coupling and duck tape. If you are buying metal pipe, try to avoid galvanized as the zinc will vaporize at the temperatures the fire will reach.
I used wood as fuel. Dumped wood bits into the side of the pit, waited for the extra heat turned it to charcoal then pushed it up around the oven. If you use wood, try to avoid anything larger than about 1-2 inches. Logs take too long to turn into charcoal.
Crushed cans and any other aluminum bits I could get my hands on went in in. You will get the best results with a pool of molten metal at the bottom then feed extra metal in. As the pool gets too cold, it starts to turn into a paste before solidifying. When you see it starting to turn to paste, stop adding metal and let it heat up and fully liquefy before adding more metal.
Most scrap just needs to be broken down so it fits in the crucible without falling out or hanging where it will fall into the fire. Aluminum cans are best fully crushed and fed into a pool of metal; stomp on them a couple of times until it can’t be crushed more. Otherwise you lose lots of the metal to oxidation.
When a good bit of metal is there (I could get about 7 pounds), skim the dross off the top with a large spoon or similar instrument, and then pour it into a mold. If your crucible is shallow, push the dross to one side and tilt it; the molten metal will flow out and you will lose less of the metal in the removed dross.
When just making ingots, I poured into a cup cake tin. The first time you pour metal in it will burn off the Teflon coating. To get anything useful, you will want to look into green sand casting.
Green sand is just sand mixed with powdered bentonite clay (cat litter) and just enough water added so it holds shapes. The finer the sand you have, the better the surface detail of the casts. Setup a stand fan blowing over a tarp and drop the stand slowly in front of the fan. The fine particles you want to keep will end up further away from the fan.
You will want some safety gear: a face shield and welder’s gloves are the minimum. Wear long sleeves and full pants, preferable 100% cotton. Polyester will melt instead of burn and stick to your skin. Leather boots are also a good idea. A leather apron of the kind used by blacksmiths is a good idea. Might wear a sweater as insulation from the heat, as the pit method wastes a lot of heat and it gets really toasty.
Absolutely have fire suppression close at hand and don’t do a run when it is windy or there is a fire flag warning. Make sure someone is available to take you to the ER if you injure yourself. They don’t have to be standing next to you, but close enough that they can help.
And above all, be extremely careful and work deliberately. You can seriously hurt yourself or others and cause large amounts of property damage if you are careless. Molten metal is instant 3rd degree burns if it touches you.
There are more resources available online about metal casting for me to list, but one that I have found helpful is Backyard Metalcasting