I was inspired a little bit by the bags by Selk'Bag (http://www.selkbagusa.com/selkbag__models.aspx) and a little bit by this bag by Alite Designs (http://shop.alitedesigns.com/sexy-hotness-sleeping-bag.html). But I was mostly inspired by those nights sleeping outside when I needed a drink of water, but I didn't want to remove my upper half from the sleeping bag just to do so. And those times laying in a sleeping bag reading when my arms and shoulders are cold, but my legs are warm.
Simply by adding arms, legs, and suspender-style (and adjustable) straps, I was able to modify a sleeping bag to fit my needs.
Step 1: Gather Your Supplies
1 Synthetic Sleeping Bag (down will be very very difficult to sew - all the feathers will fluff out and your sleeping bag will lose warmth)
1 Pair Baselayer Pants (synthetic, wool, whatever you like really. And whatever size fits you)
1 Pair Arm Warmers (these must go beyond the elbow - a lot of arm warmers are really just wrist warmers)
Super Glue/Fabric Glue/Hot Glue (Maybe)
Elastic or Webbing (Approximately 2-3 yards)
Velcro Strips (8" total)
Needle and Thread
Hopefully you already have your basic sewing supplies, so you won't need to buy them. As for a sleeping bag, I bought an inexpensive new one just in case I ruined my old one. I recommend finding a sleeping bag at a thrift store, on Ebay (what I did), or using one your currently have, as long as it isn't a really expensive one.
For the base layer bottoms, I ordered a pair online that were on sale. I wanted them to be synthetic so they would dry quickly. You could also use cotton leggings, or any type of pants. You will be cutting these apart, so don't use your favorite jeans or vintage long underwear. You could also opt for pants that had feet built in, footy pajama style.
I thought about crocheting arm warmers for this project, but because time was the limiting factor I bought a pair online that fit my needs. There are so many types of arm warmers out there, feel free to get creative.
I used elastic for the straps and it is a bit too stretchy. I would recommend webbing for solid and supportive straps.