edit: I figured it out. below is a better sample gameplg.c with multiple cheat entries (rename gameplg.c.txt to gameplg.c). the codes are from Fantasy Life and are from here http://gbatemp.net/threads/ntr-codes-fantasy-life.405886/
(the hp and sp codes dont work as they require corrected values)
just edit it to your liking then replace the gameplg.c in the source folder with the edited one and doubleclick startenv.bat and type in build.bat and hit enter and the cheat.plg will appear in the same folder as build.bat. you'll have to install python 2.7 and devkitpro for it to work though.