I've done this twice for both offspring's weddings.
Staggered sowings, with some direct sown and some in trays or modules help. You can sow sweet peas shortly, and then again in Feb. Be ruthless about thinning so that you have strong healthy plants. Support the plants too, to get straight stems. Maybe buy in some plug plants next Spring (Sarah Raven has great choice, but relatively pricey and variable quality, Crocus surprisingly good value for ammi which is brilliant for weddings).
Remember foliage, and sow some green annuals, florist's dill is great, and you can use sage and rosemary.
Don't underesitmate the amount of time it takes to cut, condition and arrange, get lots and lots of help. And you need a helluva lot of flowers. Work out how many arrangements you plan to do, and calculate the number of stems you need. Sort out vases etc well in advance.
Second the book suggestions above and Sarah Raven's Cutting Garden is also v useful.
Finally, have fun doing it! For me this meant being v organised and asking for and accepting help.