If the intention of the croutons is for soup, go with melted butter, for salads, go with the olive oil. You can also use a 50/50 mix of butter and olive oil whisked together.
Preheat your oven to 190° C (375° F). Using a knife, cube the bread, crust on or off is up to you.
Drizzle a little of your olive oil or butter onto your baking sheet. Add your bread, bunching it close together in a single layer. Drizzle remaining olive oil or butter on top, add a heavy pinch of salt and herbs or your choice if using, and toss lightly to coat. Spread the croutons out so that they are no longer touching.
Place the baking sheet in the oven and bake for about 5 minutes. Remove the tray, carefully toss and stir croutons. Bake another 5 minutes and check for doneness. Depending on the size of the bread and how browned you want them, they may need another 5 minutes or so.
When the croutons are done to your liking, remove them from the oven and cool on the pan. If not using right away, store croutons for a few days in an airtight container.