You will need a 3D modeling program. Right here on Rosity there's a forum called modeling. On their main header there are lots of links to free modeling programs. I would suggest you start with something like Anim8or or Wings. PhilC sells a tutorial CD that also includes making clothes for P5. Very good.

FREEBIES! | My Gallery | My Store | My FB | Tumblr |
You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.