Thanks for the question! That’s always a tricky one to answer because in short I’m not familiar with any systematic way to learn hand lettering (though I’m sure calligraphy would help). I’d say a very important practice to adopt is to keep an eye out for interesting examples of lettering and typography and document them whenever you can. At the start it’s going to be a lot of imitation and practice while you get a feel for proper proportions and learn the various anatomies and styles of letters. As you keep studying letterforms you’ll develop your own “vocabulary” and repertoire. A great reference is “A Visual History of Typefaces and Graphic Styles”. Best of luck and have fun!