1) Click the Application button (with the R at the top-left corner of the user interface) > New >Family. In the New Family - Select Template File dialog box, click Profile-Mullion.rft. This opens a new file using the template for mullion profiles.
The template consists of two reference lines; the vertical ref line represents the center of the mullion while the horizontal ref line represents the face of the curtain wall. Everything above the horizontal line will be behind the face of the CW and everything below will be in front to the CW.
We'll draw a 3" x 6 1/2" mullion with a 1" cap. A 1" wide glass pocket on the left side (as seen from the front) and a 1 1/4" glass pocket on the right.
2) Zoom in to the intersection of the reference lines then click the Line button in the Home tab's Detail panel. Use the Rectangle tool to draw a 3" x 7 1/2" rectangle. This is the beginning of the mullion profile to be used in your curtain wall.