Clip your hair up and do it in layers. Do a section, the let down another layer of hair and do it – and so on until you are finished. It is easier dealing with smaller portions at a time.
If you need to, you can brush the top layer up straight up over your head and iron it flat, then just touch up the roots if needed.
It is also easier if you are able to use both hands so you aren’t stretching across the back of your head.