From top to bottom: Sticky Piston, Netherite Block, Air, Cauldron with Lava inside it. You also need some form of redstone to power the piston to activate the forge, like a button or lever.

Toss an iridium ingot (don't worry, they're lava resistent) and your netherite tool of choice into the lava (make sure you don't pick them up) and then activate the piston. Particles should spray out and a sound should play, and then your iridium tool will rise up out of the lava and hover there until you pick it up. Congratz, you now have an iridium tool!