Frames come in two variations: metal or wood. Either way does the job. Either can be purchased as ready-mades (already put together), or a sections (where you buy pairs of lengths). There are advantages and disadvantages to both. Either way, the least expensive route is to avoid having custom frames built. The more you can do yourself, the better.
The frame serves to hold the frame package together, to seal the work from the elements, and to visually separate the piece from its surroundings. Frames should not be obtrusive, but should complement the work.
Some are sold as complete packages, with a pre-cut mat, glazing, and backing board.
Metal frames are almost always bought as sections. You get a pair of lengths, one for the horizontal and one for the vertical. Most places will let you order in fractions of an inch, some only at full inches. In a brick and mortar store, they are usually sold in lengths of full inches. This profile is deep enough to fit a standard frame package of glazing, mat, mount board and backing board.
Metal frames come in many different profiles (or styles) and can come in a wide variety of colors. The most common profile is shown below. Colors should be restricted to black or silver. Occasionally white is used. This frame gives a very clean, elegant look.
Metal frames are quite easy to assemble. The only tools needed are a screwdriver. A two-part corner piece is inserted into the channel inn the rear of the molding, the tightened up. Assemble three sides, then slide the entire frame package in. The fit the last side. There are a couple of different types of attachments for the picture wire. On simply pops into the rear cannel. Another is inserted and then tightened with a set screw. Put these in ½ of the way down from the top if the piece, then attach the wire.
Wooden frames usually come as ready-mades, or pre-built in standard sizes. Some are available as sections, like metal, and some can be ordered in custom sizes. Like metal, wooden frames are available in a wide variety of styles and finishes. The most standard (sometimes called a museum frame or molding) is squared, with a ¾″ face. The depth varies, allowing for thicker pieces or frame packages, as well as floating (with spacers being used to separate the glazing from the piece.
Like the simplest metal moldings, this profile is simple and elegant. Also like metal, keep to natural (clear) finishes, black, or white. Maple, cherry, and walnot are commonly used.
Wooden frames usually take a bit more effort. The frame package is placed in the frame, and then secured to the frame using points. These are small, flat tabs, which a sharpened end that is driven into the frame. The easiest way is to use a point driver or gun. The wire is attached to either screw eyes (small-sized frames) or strap hangers. Screw eves are screwed into the sides of the frame, and strap hangers are attached by screws. Msot often these should be pre-drilled.
It is entirely possible to build your own wooden frames. With the correct tools and a bit of time, they can be built quite quickly, and are far less expensive than buying frames. A couple of methods are described here.