Here's what I did:
I removed all the grapes from the stems and rinsed them and patted them dry.
I put them on a stoneware baking sheet (I'm sure you can use your metal pans too but mine was in the dishwasher).
Spread them out evenly and bake at 200 degrees F.
I checked on them after an hour and moved them around.
After another hour, I checked them again and sprinkled them with sugar. I barely sprinkled them. I wouldn't go overboard because they do concentrate and are super flavorful. Just keep checking them until they are shriveled.
I removed them from the oven and left them on the baking pan overnight.
This morning they turned out beautiful!