Using these instructions will be much less expensive than buying a quality sleeping bag, but if you've already got one that's available for cutting up, go for it! They can work great. Do a search here and you'll find quite a few posts from folks who've done just that and have some instructions on how do go about it. You can even design it such that the sleeping bag is still fully usable as a sleeping bag when not used as an underquilt.
Enjoy whatever DIY project you choose!