When you have a partial pot of lead that's already at temperature, drop the ingot in and let it melt and the pot return to casting temp. For an empty pot or a led bar too large to fit the pot, I've often just used a propane torch and melted either directly into the pot, or into my Lyman bar mold.
I've got a large fluted bar of lead that I've had for a few decades. It's too cumbersome to use very often. I've cut off pieces with a splitting wedge and mason's hammer, but the propane torch is a lot easier.