They are relatively easy to make. I used about 1 liter (4 cups) of fruit mash for one baking sheet. For the Redcurrant mash I put the mash into a pot and pour a little less than 1 cup of water over it. I then added sugar to taste. Since I was using leftovers from jelly making, the mash was already slightly sweet, so I didn't need a lot of additional sugar. Just start with 1 tablespoon and let that dissolve at a low heat and then taste it to see if it needs more sugar. I've seen recipes without sugar, so I guess it all depends on taste. I also like to squeeze a little lemon juice in there to give a little tang to the taste. I let this simmer for 10-20 minutes making sure all the fruit was well mashed and the whole thing has thickened. It's good to give it a last taste to make sure that it's neither too sweet nor too sour. Then I strained it. I did that with the Red Currants because I had all the little branches and stuff in there. When I used Rhubarb mash I didn't strain, but that makes more of a cloudy fruit roll. It can be good to use a stick blender on the fruit mash to make sure it is well pureed.
This is then poured on a baking sheet lined with baking paper or silicone. The thickness is 1/2 to 1cm (1/4 to 1/2 inch) thick. This is put in the oven on a low heat 60C (140F) until it has dried and can be peeled off. This can easily be overnight in the oven on that heat. I don't have a dehydrator, but this would be perfect for that.
When I do the Rhubarb mash fruit roll I also add an apple to give it more substance. In fact most fruit can be used and I think Blueberries would also make a very nice fruit roll and those probably wouldn't need any additional sugar.