You'll need to locate the appropriate parts, which can be found on any '89 or earlier 3-door hatchback (obviously without passive belts :).  Here is what you'll need:

• New inertia reels & belts ($25 per side)
• New receptacles which match the above reels & belts ($10 per side)

NOTE: When removing the driver's side receptacles from a parts car, be sure to cut the little black wire as far from the assembly as possible to make future reconnection easier.  If you trace is back far enough, you should find a small 1-pin connector, meaning no cutting.

• Six mounting bolts - per side one for the reel, one for the base of the belt, and one for the seatbelt pulley
• A-pillar trim pieces and the two trim clips to mount them ($5 per side)
• B-Pillar trim pieces and the two screws to attach them to the pillar ($5 per side)
• Rear side panels (you can modify your existing ones, but it's easier to get replacements) ($45 per side)
• New headliner shell and material & glue to recover it (if desired) ($50, excluding new liner material and glue)

The amounts in parenthesis reflect the amount I paid for the items, for your reference

These parts should all be readily available at any junkyard, and won't set you back too much.  The worst part of the job is replacing the headliner, which aside from being difficult can also be expensive.  Chances are you won't be able to find a good one at a junkyard in a matching color, and even if you can getting it out of the parts car intact is a real challenge.  I suggest simply planning on reconditioning it before installing it in your car - doing so is hard work but not expensive if you do it yourself.  $50 in materials and a sunny day is all it takes.  If you do opt to have it done professionally, I strongly recommend delivering your car and having the shop do it all - yes, it'll cost more but you'll know it was done right, especially the sunroof panel (which can prove troublesome)..  

When removing the seatbelts from the parts car, make note of the locations of the bolts.  When you return to your car, you'll need to locate those places and remove the rubber plugs - all the holes are there, tapped, and ready to go.  Pull out the plugs, punch out (they're perforated!) the appropriate holes in the carpet, and bolt it all up.  You may wish to use some solvent followed by WD40 to clean out the holes - debris and corrosion can foul the threads.  You do not want to ruin these threads - they cannot be easily retapped or otherwise repaired.

Be sure to remove the passive belt ECU under the rear seat, as it will disable the blinking warning light on the instrument cluster.