The only thing you have to know is that you're changing the profile of the bar, but leaving the volume the same. You can beat it out into foil, or forge it into a sphere, but it'll still be the same volume.
Step 1: Upsetting Steel bars
Heat the end of the bar, or the bit you want to be wider to forging heat. Place the bar end-down on the anvil and hit the top, like hammering in a nail. If it cools down too much, put it back in the fire until it gets hot again, then carry on. Upset the bar until it's at the desired thickness.
You can use Upsetting to put bulges into a bar, or to make rivets, nails, bolts, ETC