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The preferred method is bending with heat (steam) or laminating thin wafers to create a larger piece of trim. This is not always doable, so cutting kerfs has its place.

Cutting several kerfs (slices) on the backside of a board allows the material to bend. It is accomplished by setting the saw to a depth of 1/16" less than the thickness of the wood and cutting uniformly spaced slices into the backside of the piece.

Try a couple of test cuts on some scrap before trying it out on the final material. This is more for saw cut depth than anything else.

Layout the kerf lines 1" apart (½" or 3/4" makes a smoother turn but is a lot more work). If using a hand saw or skill saw, use a straight edge guide to cut the kerfs. A radial arm saw works best and is the lease labor intense method.

Support the piece you are cutting (rigid board material under the entire length of the piece) prevent it from breaking at the kerfs.

Make the end cuts after you have dry fit to your project.

Apply wood glue to inside surface (kerf side) of the piece and carefully bend the material to fit. Sometimes you need to "mist" the smooth surface of the wood with a little water to help it bend.

You'll have to secure the final piece in place with a clamps or tack it with brads until the glue sets.

Suggested Reading:
The Complete Manual of Wood Bending: Milled, Laminated, and Steambent Work