I have just made some rudders and offer the following method, there are other ways.

1. laminate together some good quality (no knots) western red cedar
2. plane section of board that is in the water to roughly a naca 00xx section with blunt trailing and leading edges, leave top section square cause it's in the case
3. apply epoxy and microfibre bead to leading and trailing edges (it's tougher than timber and will handle dints better)
4. shape board to more closely resemble the desired section shape (naca 00xx) making sure to leave about 5mm of hard epoxy at the leading a trailing edges, sharp at trailing edge, elliptical at leading edge. finish should be sanded, not planed
5. cut board profile to desired and class legal shape, fine tune bottom and corners with sanding
6. apply a layer of 120-160 gsm fibreglass woven mat (not chopped strand) with epoxy (not polyester). the glass should wrap around the front and finish well behind the trailing edge. do not make it too wet, should be able to use an old credit card to squeegee the resin around evenly and get rid of excess, but don't leave it too dry. if you know how to use peel ply, that is preferred method over squeegee.
7. trim glass off roughly and give a light sand all over
8. apply an adequate coat of bog (epoxy and microballoons or similar)
9. fair with a small torture board only (an mdf board with sandpaper attached about 1 foot long, 5-10cm wide, cut a sanding block in half and glue on top for handles) fair to, but not through, the layer of glass
10. if needed, keep applying bog until happy with fairness (in my opinion, fairness is much more important than section shape) final touch-up fair can be done much quicker with car bog from any hardware store
11. paint with 2-pack primer and 2-pack gloss (don't waste time with the single packs for boards, the new teflon paints are too soft and get shredded in the centreboard case, probably best to get a professional to do the actual painting, the fumes off 2-packs are really bad, I felt off for over a month afterwards, I didn't buy a gas mask that was suited to the paint but thought close enough was good enough, read the label)

notes:
- if you intend to put holes or fittings in the board, make hard points for them with a modified epoxy and microfibre before apply the glass.
- get good dust masks and keep them clean, western red cedar dust isn't good for the lungs. good quality, proper fitting, dual elastic band disposables are fine. don't waste time with the cheap and nasty single banded ones, I've blown black snot (excuse the language) for a week after sanding carbon with one of those on!!! Not good...
- I hope I haven't scared you off it, if you give yourself enough time and take the proper precautions, you should be fine. With all boat work, attention to detail is important.

As you can see, there is a fair bit of work involved in making new blades to a semi-professional standard. I enjoy doing my own because I'm sadistic. I would certainly repair my boards in the future, but probably not make from scratch again, unless I could female mould them.

I much prefer sailing over building.