A nicely fitting miter joint is very rewarding. Radial arm saws allow the user to swing the saw arm 45 degrees left or right to cut miters. It seems like a good system until you begin to use it for something precise, like a picture frame. Swinging the arm to the right allows a good view of the material and works pretty well.
Step 1: The problems appear
When you need to swing the arm to the left there is no table remaining for material support to the left of the blade. The operator cannot see any marking lines on the material and he may need to reach under the motor to secure the material against the fence.
Step 2: Make a miter guide
The miter guide shown here is not original with me, but the basic idea came from a how-to magazine several decades ago. This one is a little small, but adequate for smaller picture frames, etc. I used a rectangle of 3/8 inch plywood. Two cleats of 3/8 inch plywood are glued to the rectangle so they are exactly square to one another, but 45 degrees off of the direction of the sawblade's direction of travel.
Step 3: To use it
Make the back edge of the miter guide even with the back edge of the saw table and clamp it to the table. Raise the sawblade off of the table a little so you do not saw your guide in two.
Step 4: Another way to use it
Move the guide to the side for even more clearance between your hand and the motor. Here the guide is resting against the fence. For proper use, the fence should not be higher than the thickness of the material used to make the guide.
Step 5: A bigger, more substantial guide
This guide is positioned and used just like the view in the last step, except that the fence will need to be removed to make space for it. It is made from 3/4 inch plywood or chipboard. The rectangular base for this guide is 13 1/2 inches by 30 inches. 13 1/2 inches is the distance between my saw's blade and the left edge of the table. The fences on the guide are about 3 inches high. The black arrow indicates the direction of the blade travel. The red arrows indicate approximately where to clamp the guide to the saw table.
Because of the way this guide works, the left member of a miter joint is cut from the right side of the guide, and vice versa.
If you wish, one of the fences could be made slightly adjustable, just in case your miters are not quite perfect the first time you use the guide.
(I would have shown a photo of the guide I made and have used, but my guide has disappeared and I cannot find it, so I made this illustration with Google Sketch-Up.)
Step 6: Own a table saw?
This is the same guide, but adapted for a table saw. Notice the graphic shows a bar of steel for the table saw's miter gauge slot screwed to the underside of the guide. If loss of cutting thickness is an issue, the rectangular base could be made of metal an eighth of an inch thick. The bar of steel for the miter gauge slot is usually 3/8 x 3/4 inch. Place it so the lower edge of the guide (as shown in the graphic) is very close to the blade. Be careful to make the bar of steel parallel to the edge of the guide.