1. First preheat the oven to 110 degrees Celsius. Set the oven temperature to about 110 degrees Celsius and preheat for 15 minutes. Add the grapes only after preheating the oven, otherwise it will affect the quality of the raisins.

2. Wash the grapes and remove the stems. Wash the grapes under running water and remove the larger stalks with scissors or your fingers. Discard the bad grapes.

3. Grease 2 rimmed baking sheets and spread grapes evenly inside. Grease 2 framed baking sheets with a very small amount of vegetable or canola oil, then spread the grapes evenly on them. Try not to let the grapes touch each other.

4. Roast the grapes in the oven for about 4 hours, or until wrinkled. The raisins must be dry enough, but not so dry that they are shriveled. Check the grapes from time to time during the roasting process to avoid mistakes. If the grapes are dry enough, you can remove them even if they have not been baked for the full 4 hours. The exact length of baking depends on the size of the grapes and how crinkly you like them. Larger grapes will take longer to dry.