there aren't as many parts on the MoW classic pack compared to the stubby, so it's pretty easy.
first, did you get the template for the holes for the nozzles? cut out the paper template, put it in place and mark where those get cut out. i used a small drill bit on my dremel, on the inside of the line drill a series of small holes. once you've got enough you should be able to push the center out. then take a drum sanding bit and smooth the edge. make sure the nozzles fit in the holes snug but not so tight that they're difficult to get in there.
the rocket assembly is pretty self explanatory. slide the collar piece into place on the upper part of the rocket, then on the lower part put a little two part epoxy in the recess and slide the two parts together.
there's a larger collar piece that you can just epoxy to the top of the jetpack body.
attaching the rocket is where things get a little more difficult. what i did was i cut the part that slides into the main fuselage, then i took a trip down to ace hardware to get some pvc plumbing fittings. you're going to need a threaded coupling and a matching plug. now these don't thread all the way in, they get tight well before they're all the way twisted together. i think i got the 1", but you should double check to make sure you get the right size. i cut a recess into the base of the rocket to fit the plug. once it fit in there snug i held it in place with epoxy. now the coupling is going to be too tall to fit into the rocket cradle on the body of the pack. so thread it on the plug as far as it will go, then mark the depth on the coupler and cut a couple threads deeper than the mark. then a little more epoxy on the coupler, center it in there, and let it dry.