Cut your bread into cubes any size you like, they don't have to be perfect and they don't have to be all the same size!
Melt butter in a microwave-safe bowl, once melted add the onion powder, garlic powder and salt - with a whisk give it a good mixing - if you let it sit before pouring over the bread be sure to give a quick stir again.
Place all your bread cubes in a large mixing bowl, pour your butter mixture over the bread cubes and stir to coat.
Line a large cookie sheet with parchment paper and spread the coated bread cubes out as best you can in a single layer - again we aren't looking for perfection or for them to not be touching - this way each crouton will be just a little different in texture and crunchiness!
Bake in a pre-heated 350° oven for 20-25 minutes. I like to give mine a toss about 10 minutes into the baking and then every 5 minutes or so after that until they are done to my liking. We eat a very soft crouton with light toasting - you can totally bake them longer to a crunchier crouton if you like!