L.L.
My 1.5 year old tries every time she goes, and then I "finish up" for her. My 3.5 year old (who I started the same way as her little sis) does it herself, and does not want my help, but I do occasionally see skid marks in the underwear (I don't make a big deal out of it, I just tell her to try again and get a clean pair of underwear).
If I were you, I would require that she do it herself every time, and that you will help her after she has given it a real try. That way, she can gain practice and eventually confidence in her abilities. Some kids really freak out about not doing it perfectly because they don't like feeling soiled. Knowing you will back up her efforts may give her peace of mind and help her to get past the emotions and work on the skill development.