Clean the frameless picture frame with glass cleaner and a soft cloth. This allows the glass paint to adhere well.
Tape a paper design -- one from a coloring book works well -- to the backside of the glass with masking tape. Position the design so it is at the corner section of the frame. Another alternative is to use a stencil or draw a design freehand. Tape the stencil to the front of the glass. A monogram, single flower or a geometric shape are all simple designs you can draw freehand to decorate the corners.
Paint the design with the glass paint and allow it to dry according to the directions on the bottle. Reposition the paper design or stencil and paint the other corners in the same fashion if you desire.
Remove the stencil or other design once the paint is dry.