In reply to Roberttaylor:
there's a good overall guide here-
http://bushwalkingnsw.org.au/clubsites/FAQ/DIY_RNCSB.htm
as far as preventing the half bag riding down, I think elastic's your best bet.
For half length bags, the Alpkit option is £70. I've done previous research on the internet for making a full bag and, all in, the purchase of the down filling, outer and inner material, thread, elastic, zip and getting access to to the tools to actually make the bag would probably work out much more expensive than actually just buying the half bag from Alpkit.
That said, once you've cut your teeth making the first half bag, you'd have the skills and some left overs to try a full bag, which is probably when the real DIY money saving comes in.
FYI - it doesn't seem that easy to find 100% goose down stuffing on the internet unless ordering from the US. Also, if I was making a half bag, I'd just buy an OK quality synthetic bag, cut it in half, re-stuff it with goose down and sew it up - bish bash bosh - bob's your uncle!