That said, it`s worth it. Your basic needs are a place to work (remember that heavy beating will carry a way in any building) , lots of patience and a sense of rhythm. (Lily-the-Pink is fine for most hammering jobs.)

For many things you need little more than a rubber mallet and a curved surface, even when working steel. For serious work you`ll need steel hammers (a set of them) and make sure they`re polished, because any pits or scratches will be printed on your work many times over. Wood or sandbag is good for shaping and dishing plate. The time that you will be needing both a steel hammer and a steel surface to work on, is in the last phase of hammering, when you work out all the irregularities (planing?) before you start sanding and stuff (what`s that stone called again in English? puimsteen).
Of course, anything thicker than sheet metal and you`ll be wanting to get an anvil.

1mm is good enough, although you might want to add a few thicker pieces for structural integrity, but that`s entirely optional. Plain steel is good. Alu is evil. Stainless steel limits the possibilities.

Do you have any idea what you`ll be making? Ancient armour tended to focus more on the length of the limbs (the basic LARP approach: just stick your limbs through some pieces of tube), while more modern armour focused mainly on the joints.

Lamellar shoulder guards work well with most kinds of armour. They`re fairly simple to make too, and require extremely little finishing, so they`re a good starting point. Simply bend strips over your knee and connect them with two bands, for which you`ll be drilling holes. The closer you put these two together, the more flexible the whole will be, the wider you put them apart, the more they will stay 'in formation'. (Use only one band and they will turn in place.) Don`t put them more than 1/3 of the lamel apart, though, or you`ll create openings where you don`t want them. The shoulder guards I made for my lorica segmentata were rather large and just kinda hung down from the top plates, with my arms moving around below them. The other pair I made were tied to my arms and were supposed to move with them. If you want flexibility, your lamels shouldn`t be too wide, nor too close together. Just a small overlap will do.
Make sure all edges are smooth. They`ll only cut you and others later, first they`ll cut any costume and weapons they get into contact with.
Instead of rolling or upsetting the edges (only the outer edges, that is: the short sides of the middle lamels, and the three outer sides of the first and the last) you can sew some cloth or leather over them. (For which you would need to drill some small holes.) Adding some cloth or leather to the underside will protect the rest of your costume from wear (and grease?) as well as making them more comfortable to wear and dampening sound when worn in combination with other armour. In fact, lamellar and splint mail can look very good, if properly furbished with some attractive fabric.

That`s the basis of lamellar. Splint is even easier, as it doesn`t even need to be bent. In case you do want to bend splints or other parts lengthwise, you`ll need a hammer (wood or nylon, as that`s less likely to damage your material; you`ll need most of the patience I mentioned earlier in the finishing: working out irregularities, sanding and polishing) and a solid curved surface. Wood is good. Not in the least because it can be shaped to your wishes. If you`r not into wood, simply pour some concrete into a piece of pvc piping or a smooth can and wait for it to harden.
Add a piece of half-pipe to the shoulder guards on either side to cover the rest of your upper arm, and make some (splint?) bazubands. Tadaa: arm protection!

Oh, and you can buy sheet metal pre-cut to your requirements.

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IRL: Edwin Hofstra

- mostly crewing at the moment

Yes, planishing and pumice. Planishing is done between two solid steel surfaces. I have the planishing hammer here in my kit to prove it. I`ll admit that these are silversmith`s techniques (I`m  trained goldsmith myself, but I was able to use a friend`s silversmithy to make a lor.segmentata and a nice red bronze (tombac) Thracian helmet) and they`re probably a bit archaic, as silver- and goldsmiths tend to be less industrialized than the steelworks. - We were even taught the theory of using cats`tails (a rough skinned weed) and sharkskin in case we`ll find ourselves in a sandpaper-less society after the next big one.

The pumice is used in the way described in Nocturne`s reference. It is a preferred method for hollow surfaces, because it adapts to the curvature (by erosion). Given that silver reflects 99% of the light that falls on it, sanding with your fingers and a piece of sandpaper just isn`t perfect enough. (And there would be no end to your collection of sanding rods if you were to use those for all the - often multidirectional - curves of silverware.)

You probably won`t need it much for armour, though, as the hollow is usually the inside and it`s perfectly alright to leave that side 'rough' (after planishing, of course).

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IRL: Edwin Hofstra

- mostly crewing at the moment