This is, or was, a lot on this in the old format. I use a very nice Durston open mold, 2 machined two piece, wire on one side flat on the other side, molds and delft clay. In all cases I make sure that the scrap is clean and don't use anything that looks suspect. I also run a magnet around the little pieces. Regardless of mold, the scrap is melted in a small whip type crucible that has been well coated with borax. Heat it with a good clean flame with enough heat to do this quickly. Once the melt is fluid, hold it for a moment and then I usually sprinkle a little more borax on the melt. I use a salt shaker to do this. Using a graphite rod sharpened in a pencil sharpener, run it around the melt to dredge out what ever will come with it. Make sure the melt is fluid again and the carefully pour it into whatever you are using for a mold directing the flame on the pour stream and sprue button slowly moving the flame away from the button as it cools.
I coat my open molds with a light layer of 3 in 1 oil. Heat it a bit and then pour. t takes some time to figure out how to keep the pour together. You might try elevating one end of the mold a tiny bit to get all the pour going in one direction. If I am using the two piece steel mold, I open it up and heat it well to drive off any water that condenses from the gas and air mixture, then using a gas only flame, coat the inside of the mold with soot. Put the mold together making sure the mold halves are lined up correctly (use old oven mitts), heat the mold a bit more and then pour as described above. Delft clay is a lot of fun and a separate topic but still melt as described above and pour.
Once you have removed your ingot, regardless of shape, remove all the burs and other sharp paring line edges. I do this on a lapidary sander with 220 grit belt. Anneal again (this may not be necessary but I always do it), then forge the ingot with a forging hammer in all directions front and back. I usually forge it in a rough approximation of what the final shape might be. Anneal again and then start rolling. I only roll in one direction keeping track of what end goes in first with a little sharpie mark. Anneal often. If you see a fissure or sharp opening, file it out or at least even it out. Did I say anneal often. I don't always pickle after annealing, but just use water to cool it. Pickle can do a lot of damage fast to your rollers, so be careful. There will always be some loss, so roll bigger than you need. Clean up the rollers when you are done. I use simichrome to polish and work it in using a dust free cloth and polish like you would polishing shoes. Then apply a light coating of 3 in 1 oil. Keep your mill covered when not in use but don't seal and lock in moisture. I use an old pillow case. If you would like to cast an odd shape like a triangle, get pieces of square steel tool rod and lay them out on your anvil in a shape as close to what you want as you can get to with the steel rod. Hold the pieces of rod in place with small magnets and pour. Now you have an odd shaped ingot that can be forged or rolled into your odd shape. This is a lot of fun. That's it, more when I know it, good luck...Rob