Fabrication of Reducing Miter Bend

Haven't given this much thought but, if you're talking about pipe, wouldn't you make it the same way as a regular miter bend except start with a tapered pipe?

This is the subject of the overall mechanical drafting topic known as "developments and intersections". There is more than one way to go about it, so you need to be more specific about details of the reducing miter bend you are looking for. Most likely it would be formed from various conical segments.