The headdress (wimple or habit) isn't that hard. First you'll want to measure your head with a measuring tape around the point where you will wear the wimple. Then add a few centimetres on each side. After that, you'll want to cut a strip of white out with that length, but twice the width so that you can fold it over in half to have a clean edge (to be the side that faces down when on your head).
Once you do this you'll want to sew snaps on each end of the white strip so that they snap together and stay on your head.
The blue part can be made by tracing your own pattern on a newspaper
in a sort of half-shape of what you want the final product to be like lj-user aneris has kindly drawn a diagramme of:
(whole post here: http://community.livejournal.com/burijitto/22813.html
so yeah you basically trace your pattern onto your fabric with the non-diagonal side against the fold of the fabric to create an even shape.
To attach the two together sew with the tops of both the white strip and blue piece aligning most of the way to the snaps (leaving some space) and then just flip it over and you have a clean seam.