Use Simplicity pattern 2518 as an easy sewing pattern for a nun's habit. This pattern is available through retailers such as Jo-Ann Fabrics and Amazon.com. Butterick produces sewing patterns such as 3820, a pattern for a unisex robe and collar, and Butterick 4542, a pattern for a children's robe.
Simplicity 4795 and 4213 are patterns for Nativity costumes and head pieces, respectively, that can be easily altered into a nun's habit. Find these patterns at Simplicity.com
Make an easy nun's habit without sewing by combining a white t-shirt with a piece of black fabric or a black t-shirt. Place the white T-shirt over your head but don't pull your head through all the way so the fabric circles around your face. Next cover the top and back of your head with the black T-shirt, and pin this fabric in place. This gives the look of a nun's habit easily without sewing.