for which version? if you want to make one for 5.1 I recomend just making a copy of the default noteskin (in the newskin folder) and reading the docs inside of it
as for 5.0, just download one of my noteskins from
zettbou.co.uk and read the stuff in there and mess around, if you have any questions you can post them here or catch kyzentun or me on irc at irc.freenode.net #stepmania-devs