I found myself in exactly the same position as you with approximately 5kg of white and red grapes from the garden.
I'm no expert by any means but hears what I did. I destalked the grapes and put them in a fermenting bucket,then washed them.I then mashed up the grapes with a potato masher and added 4litres of supermarket grape juice that i had in the cupboard as i was worried that my grapes may be too acidic I then made up to 15litres with water.
I took a sg reading it was 1045 I then added a two kg bag of sugar straight in not syrup and three crushed campden tablets gave it a good stir and left for 24hrs.
The next day to my amazement all the sugar appeared to of dissolved as after a quick stir I had an SG. of just over 1090.I then added 3 teaspoons of pectolose,3 of yeast nutrient and a sachet of ritchies red wine yeast,i then fermented this on the must for just over a week stirring when i remembered.
I then strained through muslin into 2 and a 1/2 DJ's topped up with grape juice and fermented out.
I am just about to give this its first racking it looks like red wine it smells like it not sure about the taste at this point.
This is the first time I have done this a would welcome any comments.
Cheers Graham