depends on what software you have. I would recommend
downloading inkscape (incredible free open source drawing program). it can edit pdfs very well (it is a vector, not a pixel editor, which makes it well suited for the job) and it has alot of online tutorials to help you get starter. but remember it is a very advanced and powerful program, so dont expect to be able to figure it out in the first day (or week. or month (jk)).
other then that, there is adobe illustrator, but that is crazy expensive. also,
this is a bit simple then inkscape so you might want to try it out.