You can use any kind of string or thread, but keep in mind that this will make it harder for the hat to stay on your cat's head. Yarn does work, but it is harder to use. I would say to just use string.
The elastic band is only for the uppermost part of the garment that will allow you to wear the top as an off-the-shoulder garment. The instructions from scratch for the...
Yep! Measure around your ankle with the elastic not pulled tight, but firm. Snip with an extra 1 cm for sewing together last. When you are up to the ankle...
The next safest surface would be a room with carpeted flooring. That way if you fall it won't hurt as much. Place pillows around you to soften any falls.