To make large sheets of wool felt my friends who do this use either a large towel, lay the towel on a larg sheet of plastic, lay out your fibers, wet with hot soapy water, and roll thE whole thing up and roll it back and forth over and over, and over. Some use a noodle as a core. Another friend uses bubble wrap, or a bamboo shade. The towel and plastic seem to work very well and are least expensive. You can prefelt it slightly before rolling it to help it hold together better. Use a gentle touch until it has begun to hold together. It is exhausting work, get the kids to help, take frequent brakes, your back and arms will be killing you. Rollin in one direction, Unwrap it, roll it in the other direction, and repeat rotating it every so often so it becomes an even felted sheet.
In Letty's book you are using roving.