Last year I was a nun for Halloween. This Mother Superior costume required a lot of sewing. The main gown of the Habit was a long black gown. I made mine, but a graduation gown or something from a witch or grim reaper could be used.
Next came the white collar. I used a pattern I found for a quaker type costume and widened the collar. You could also use white foam sheet. Attached to the collar with sew-on snaps was a long black rectangle of fabric.
For the head piece I used a white sheet of craft foam. I cut out a rectangular strip that wrapped around my head to just behind my ears on both sides. To this I sewed another piece of the black fabric cut into a large square. I stapled elastic to the two ends of the foam strip so it would fit around my head.
The finishing touches included a rope I tied around my waist and a rosary I made myself from beads I already had. The big cross was made for me by my grandmother a few years ago.
Some black hoes and black shoes and you are done!