Five years ago, depressed at wearing out sails at a suit a year and feeling the financial pinch, I started to make my own sails. Now several hundreds of yards of cloth later, I realise just how little I knew about either of the two basic skills involved, designing the sails and constructing them.
The worst problem seems to be their sewing, but this turns out to be largely a matter of planning and dexterity. The real problem is, in fact, their design and I cannot recommend any book that is specific enough to be really helpful. One will probably start by copying professional sails, but let us start from scratch.
Firstly, read the class rules and discuss them with an experienced measurer. Decide on ratio. For the jib, decide on luff length and diagonal. A short luff means low aspect ratio and good flow at the head. Keep the clew low to close the gap at deck level. The main is more complex. I suggest that the foot should be less than 7 ft. as above this the top batten will not set well.
Secondly, examine top helmsmens sails. How full are they? Where is the fullness? Will their mast bend take it out and free the leach? How flat does their jib go? You should now be able to start thinking about shape in three dimensions. Ideally, you need a main that will flatten and free at the leach exactly in proportion to your being overpowered, but it should also be full for down wind. A bendy mast and a powerful winch to stop its bend are essential.
Thirdly, measure the bend of your mast, taking offsets to find position and amount of bend. Now make a scale drawing of the sail plan. Draw the mast curve and add to it the amount of fullness you want left when the mast has bent. I would suggest that this will be of the order of two inches at about three foot up, tapering over about eight foot into the line of the mast bend and possibly going an eighth of an inch or so inside it about two feet from the head. These amounts are for a full sail on a C section mast. They will flatten the head fairly early, leaving some fullness lower down. If you are heavy, try making the head fractionally fuller. Reduce all the measurements if you are very light. Now mark on the two foot, half and quarter height and clew positions. You have to fiddle to find the batten positions but should eventually have on paper the shape you hope to make the luff and leach. Estimate the foot shape, for it does not work out in three dimensions as it does on paper. Transfer this outline full size to a large floor, either in chalk or by pinning white wool to a carpet at six inch intervals.