I haven't DIYed it, and wouldn't have much interest in doing so, but my friend has kindly offered to do mine for me as my present!! She did her own last year and loved doing it, so it can't be all that bad if you're a DIY fan!! :-)
I know she watched videos online, and I know she's buying half the organza in China from ebay, apparently you do need a lot to bulk up the arch (which she bought on ebay too I think, just a bog-standard metal arch)
She also mentioned garlands - I think she said they're about a fiver each..
It's definitely achievable if you've got the interest!
Good luck!! :-)