I fancied some and had a spare healthy extra so . . .
Preheat the oven to about 100/150. Low enough that the croutons will dry out without burning. A real range would be perfect.
LIne a baking sheet/tray with parchment and spray it with fry light or similar.
Cut sliced bread into squares (I keep the crust on)
Tip them onto the sheet and spray again.
Pop them in the oven and leave them.
After a while, turn them over and maybe re-spray.
When they are lovely and crunchy, take them out, sprinkle them with a little salt and try not to eat them before adding them to your soup or whatever.