If you mean starting from scratch, you can't really - Dr Geep has done some tutorials on using the group tool and the Poser props to make simple props but for anything else you need a full modelling application, preferably capable of exporting in Wavefront OBJ format or at least some format Poser can read. About half-way down the list of Topic Forums on the right side of this page is the modelling forum, which has links to several free tools.