Insert the screw driver into the exposed end of the 90 degree fitting, inside of the barrel, and use the large adjustable wrench to tighten the hose bib until it is positioned correctly. I usually only have to make 3 or 4 turns with the wrench until the washer has bent in slightly. Do not over tighten or the threads of the hose bib can shear right off.
This part is really what I am most proud of about my rain barrels. Many people have asked me how I connect the hose bib and what keeps it on so tight. The secret is in this image.