You don’t actually make the ROM. There are developers that make the ROMs for free. They accept donations for their work.
This is the best site by far.
http://forum.xda-developers.com/filter.php?cat=Android
Find your device section and start educating yourself on how to do it. Most phones have threads dedicated how to flash a custom ROM step by step. Use the Q&A thread to ask questions. Make sure you follow directions EXACTLY. Flashing incorrectly can “brick” your phone and make it a permanent paperweight.
Thank you for the site. I have heard about flashing the phone but don’t have the gumption to do it yet.