You cannot craft a Ghast Saddle in vanilla Minecraft
Ghast Saddles are not a standard item in the base game
If you mean a modded Ghast Saddle, check the specific mod’s recipe or crafting system
If you mean a saddle for riding a ghast in a mod, obtain it through that mod’s crafting, loot, or commands
If you want a regular saddle in vanilla Minecraft, find it in chests, fishing, trading, or loot