On the 10 minute runner, the only way we could figure to cut the batting was to piece the 12"and 18" together, then iron them so you have the actual width of the runner. Then measure that and cut your batting that width and turn your pieces right side out & slide the batting down into the runner, making sure it lays flat. Then sew across your ends & turn. If you have insulbrite or needle punch thin batt works the best. One lady did hers with warm and natural & it was a little lumpy. I didn't put batting in mine, sorry don't have a picture. Hope this helps.
Edited to add: you may need to do some quilting as per the requirements of whatever batting you choose.