Start by looking at these tutorials:
http://hjklol.mit.edu/content/visualizing-molecular-orbitals
http://www.ks.uiuc.edu/Training/Tutorials/cpmd-vmd.pdf

Checkout: projwfc.x

In summary, write an input for the post processing tool as follows:
&inputpp
prefix = 'pwscf' ! must be the same as in pw.x input
outdir = 'pwscf_output' ! must be the same as in pw.x input
kpoint = 1 ! different spins have different kpoints !
kband = 1 ! orbital of interest
plot_num = 7 ! charge-density from one orbital
/

&plot
iflag = 3 ! 3D plot
output_format = 6 ! output in GAUSSIAN cube format
fileout = "density.cub"
/


Read near the end of your pw.x output to find HOMO and LUMO orbital/band numbers. (grep “bands” and “occupation”). Run your script with pp.x.