If you have time (approx 6 hrs), a chair, fabric, measuring tape, a TON of straight pins, and a sewing machine - you can make a slip cover. Trust me - I am by no means a skilled seamstress and I am pretty lax when it comes to technique, and over the weekend I made the slipcover pictured above (woohoo!). UPDATE: I started cutting/sewing my second slipcover yesterday, and it'll probably only take me half the time (3hrs) to make it (and it's looking crisper)
Step #1: Find a chair:
(mine were 2 for $50 on Craigslist)
Step #2 and #3: Select and measure/cut fabric:
I divided the chair into sections (e.g. back, front, arm, side), measured, and then converted those measurements into rectangular cuts of fabric (approx 3 yards, I used Waverly fabric in Country Club).
Step #4: Pin and Trim
This was the longest and most dangerous (b/c of pinpricks, ouch!) step. Following the lines of your chair, pin the fabric (inside out) where you want your seams. Trim areas where you have extra fabric, but not too much, since you need a seam allowance and wiggle room in case you make a mistake.
Step #5: Sew
Once your slipcover is perfectly pinned, slip it off the chair (mine came off easily), walk over to your sewing machine, and start sewing (note: I used heavy duty thread and a heavy duty needle). Follow your straight pins, removing them as you go.
Whenever I became frustrated with a section, I'd temporarily abandon it and start on another. I figured all the seams would meet up eventually. After I thought I was done, I slipped the cover on the chair (still wrong-side out) and trimmed/pinned and even hand sewed a couple stitches until it looked right.
The best part of the project was flipping the slipcover right side out and putting it back on the chair. It may still need a couple tweaks, but I think it looks 10x better than any floppy cover I could have purchased at a Department store.