A paper or styrofoam plate with an odd number of half inch slits around the edge is how we got started. I made 13 slits. I had to do the prep work for most of my students by wrapping yarn across the plate and up the next slit to go across the front again. Because we needed an odd number of slits, I poked a hole in the center of the plate and pushed the yarn through the hole after the final slit, before tying the ends together. Then we threaded yarn in a big plastic needle, although the fat yarn didn't need a needle, and started in the center and tying the two ends together on the back. "Under, over, under over" little voices said quietly as they tried to push the yarn under a cross thread and pull it out the other side.
Students with multiple complex needs obviously weren't ready for weaving, but they could manage to wrap the yarn around the plates and with some help, pull them into the slits. It's important to differentiate by ability levels so that children don't become overwhelmed, while maintaining high standards and expecting them to keep trying.