The first step is to soak it for at least a week. Use something like kroil, pb blaster, acetone/atf mix. You can flip the bike over and pour some down inside the steer tube.
The most important ingredient is patience. It's steel-on-steel, so it will come apart...eventually.
I saw your other photo and the stem doesn't look like it's inserted very far. That'll work in your favor. You've already knocked the wedge free, right?
Put the top of the fork in a large bench vice with hardwood blocks to prevent damage and use a large breaker bar in the handlrbar clamp on the gooseneck to twist it loose after soaking in blaster and heating it with a torch.