Making a 4" x 6" Kilnformed Glass Photo Frame  
I use a 4" x 6" metal and glass frame like this:
Back View. They come in various brands, and usually in silver or black color. Often available at Michaels, and various craft and hardware stores.

Cutting the glass:

I make mine with two 3mm layers of glass.

To do this I cut eight strips of glass 1" wide.

4 strips are 6 1/2" long,
and 4 strips are 4 1/2" long

The exact size of the glass is critical if you want it to fit properly on the metal frame backing.

Lay the first four strips in a perfect rectangle. Note from the photo to the left, and the photos below, how the seams are made so that each seam is perpendicular to the adjacent seams. It helps to have some sort of way to know if your re making perfect right angles. I lay these up inside an "L" square. Lay the second four strips in another rectangle on top of the first. Stagger the joints so the second layer overlaps the first. It may help to add some Krazy Glue to hold the layers together, particularly if you have to move your shelf into the kiln after assembly. Let it dry, then move it into the kiln. In this example there are two frames laying on the kiln shelf. I added a layer of copper leaf between layers of clear and vanilla, and added some hotworked elements and frit on top.
Firing Schedule: This may vary depending on how rounded you want your corners, or how much three dimensional relief you want for your design elements. But, a typical schedule might be to heat to a top temp between 1425F and 1490F depending on the appearance you want and the behavior of your kiln. Once the frame has been fired and cooled, run a thin rim of glue ( I use E6000) on the three front facing edges of the metal frame backing. Try not to use too much glue. Center the glass frame on the backing, and make sure it will sit on the table the way you want. Make sure you don't glue the clear glass in place, as you will want to slide it out later. Lay face down to dry. In spite of the small strip of glue holding the glass in place, I've never had one of these come apart.  
Finished fused glass frame, with French Vanilla background, clear