Mums over winter
You say "After the foliage of the plant has died back, you will need to cut it back. Cut back the stems of the mums to 3 to 4 inches above the ground." Other web sites say "First, leave the foliage on the plants until spring. Don't prune them back after frost has turned them brown." So what is best? We live in northern York County PA, where winters can get nasty cold, with lots of snow and freezing rain or ice. Last year we had one snow storm that dumped over 2 feet on us. And then we had 3 days in February when the low temps were in the teens, then 3 weeks later 3 days when the temps hit the mid 70's! In other words we have weird winter weather. PS: our mums are hardy.
Some gardeners leave stalks intact in the belief that they will trap leaves and help protect the plant over winter. Others cut them down for aesthetic reasons and apply 2 inches of wood mulch or chopped leaves over the crown and roots if concerned about winter hardiness. In my experience mums are hardy: they are rated for zones 4-8. Both approaches are fine; it comes down to personal preference. By the way, snow is a great insulator that keeps the coldest air away from the plant/roots. .