You can try this, but it will be a do or die kind of deal once you start.
You can buy joint compount (mud) at HD or Menards (5 gallons to start). Tell them you need the slow setting, easy to work with stuff. Get a 6" and 12" metal trowel, and a trowel bucket.
Mix the mud up in the bucket (a 13" long bucket about 4" x 4" or so). Use some Dawn dish detergent with each batch you stir up. The Dawn actually helps the stuff mix without bubbles and is the ancient Chinese secret of every drywall guy in the world. Use about the same amount you would for a good batch of dishes (per load).
Pull a field of mud as thin as possible across all wall areas with the 6" trowel, and finish back with the 12" trowel. You'll need to do this several times until the walls are smooth and you might need to sand some spots down if they are high, or you make them high, (a sharp knife might be handy for any humps, but be gentle)
A better suggestion to you would be to find a local handyman drywaller, and have them come out. It is probably about a 4 hour or less job for a good mud man, and it'd come close to their minimums and they'd be done. It'd be the best 300? bucks you'd ever spend, trust me.
For you, it'll take a couple of weeks most likely, depending upon the size of things and how good you are.