When I first dipped my toes into forging, I didn't heat the steel sufficiently and ended up with cracks galore, definitely a quick lesson learned. It's best to work the metal when it's glowing bright red to orange, which is around 900 to 1,200 °C, depending on the type of steel. If it starts looking dull red, you've let it cool down too much, and that's when those cracks can show up. Also, make sure the heat penetrates all the way through the metal, not just the surface. And if it's cold outside, keep your anvil warm, that'll really help out a lot.