Hello I am like you a Blythe lover, but also I covert Tangkou (of which I now have 2) and I have a few vintage Sindy and other vintage fashion dolls from the 70's & 80's.
I am no expert and like you a starter outter.
May I suggest buy some cheap Charity shop (goodwill) barbies/Bratz etc to practice on them. This my aim before I buy my Blythe or Bassak Blythe to custom.
I have been searching the internet for how. These are some and only some of the sources I have found
Such as ehow http://www.ehow.com/how_6637310_repaint-gene-dolls.htmlhttp://www.retrodolls.co.uk/http://dolls.wonderhowto.com/how-to/reroot-blythe-doll-hair-271165/
there are lots of people who have hints and ideas like http://www.thethinkingdoll.com/apps/blog/http://www.libertyjaneclothing.com/
My inspiration and wish to have the cash would be lolly's collection on http://www.flickr.com/photos/lollygomez/
or to produce like abi http://www.flickr.com/photos/38519424@N08/
I have been making 12 inch dolly clothes for a FEEEWWW years on and off. a Blythe is very similar in shape to a Barbie. I always make a mock up in cheap as chips fabric. usually from clothing I am thrownig away or charity shop finds.
I'd then try draping and fitting fabric to the doll to see how it fits & cut the shape I want. I generally use the same techniques in doll sewing as human sewing, but you have to think about the smaller areas first. forexample - a hem on a sleeve or trouser leg and even a waist bad is easier to sew with the flat fabric than trying to do it afterwards.
If you need patterns - Simplicity have a few and occasionally the appear on ebay for very little money. There are also quite a few books on the subject on amazon as well as few "Tutorials" on the web.
I would say just have a go and see where you get.
Good luck and we look forward to seeing your results.