Step 1: A Note on Concrete Filled Anvils
The trick then, is not to rely on it too much. You want to build an anvil that would work even without the concrete. It is there mainly to add weight and solidity, and deaden noise.
Step 2: Acquiring Materials/Skills
As for tools, you're going to need,
Some sort of welder,
An angle grinder,
Something to cut the sheet steel with, such as an acetylene torch, plasma cutter, tin snips(not advisable), a bandsaw, or a crap-ton of cutting weels for the angle grinder.
The main other thing that you need, is to be a competent welder. I repeat, this is no beginners welding project, you need to be able to make a weld that can be hammered on for hours without breaking.
It should be noted that I got all the steel for this thing(accepting the top plate) for free as scrap, and you could probably do the same.
Scrapyards are a great place to get the main plate. I got one that is 1 1/2" x8"x 36" for $30. The ancient, mystical redneck junkyards one sometimes finds in the woods are a great source, or ask people if they have any old doodads you can salvage steel from.
Step 3: Designing Your Anvil
I would suggest drawing out your own, but feel free to use my schematics if you like(but no plagiarism please). They're not extensive or anything, but you're a clever fellow, you can figure it out.
PS, if you ask VERY politely, I can make some better blueprints.
Step 4: Make a Cardboard Model
Step 5: Cut Out the Pieces
Step 6: Weld'n Time
Basically, you just slap it together the way you did with the cardboard model, but with steel.
Now for one of the most important steps, the inside supports. You want at least five really heavy duty legs on the bottom of the main plate, with a few more on the horn. Then just go crazy and weld rebar onto every available surface, there really is no such thing as going overboard on this step.
Post script: When you're doing anything with the top plate, remember to turn the power as high as it will go, and set the wire feed fairly low.
Step 7: Fill'er Up!
The main other thing is to make sure that you have it positioned so that once it is full you can simply roll it into place, where it will be permanently.(you can't move this thing when it's full, just don't even try)
Step 8: Make It Perty
Step 9: BEHOLD!
That day, Vulcan threw his hammer to the ground in dismay, for he knew he was bested. That day, the ringing hammers of the forges of heaven lay still in respect.
I heeded them not, but instead gazed in awe at what I had wrought, and saw that it was good, nay, it was friggin fabulous!