i'm a little confused -- are you asking how to convert an IF skin to proboards or how to convert to proboards in general? there's a forum we've got called
converting to proboards where you can find some tutorials. unfortunately, i'm not very familiar with proboards skinning, but to my knowledge it's quite a bit different than invisionfree, so it would be fairly difficult to convert an IF skin to use on proboards. however you can find proboards skins
here.