1.   Cut the pieces, then test fit them to make sure the joint is square with no noticeable gaps.

2.   The pieces must be cut accurately and fitted together so they are aligned properly without noticeable gaps.

3.   This will leave a noticeable gap between the came flanges and the glass.

4.   When the joint fits snugly without any noticeable gaps, nail the molding to the wall studs.

5.   You could, of course, butt the two pieces together, but over time the molding may contract, allowing the joint to open to a noticeable gap.