How to Convert 28V to 12V?
One way to design this into a schematic is through use of online tools such as those provided by Analog Devices, TI, and others.  Here are two links that might help:  https://www.analog.com/en/design-center/ltpowercad.html   http://www.ti.com/tools-software/design-center/webench-power-designer.html

After you get a schematic together post back here with further questions.