A french seam encloses the fabric raw edges without using overlocking. It is a beautifully neat finish for a seam especially with sheer & light weight fabrics.
The seam allowances are determined by the weight of the fabric, for example chiffon requires a smaller seam allowance than cotton poplin.
This sample has a 1.5cm seam allowance, with WRONG sides of fabric together sew a 0.5cm seam. Trim away any fraying threads.
Press both seam allowances to one side.
Now, with RIGHT sides together sew a second seam with 1cm allowance to enclose the raw edges.
wrong side 
right side
Garment Construction 