Flatlocking: there should be instructions in your serger manual. Otherwise, take a look at this book for many fine serger techniques:
Complete Serger Handbook (A Sterling/Sewing Information Resources book) (Paperback) by Chris James (Author)

For further excellent tips that might help, talk to Stuart: http://home.iprimus.com.au/seanderson/index.htm

Stuart has spent many years designing and making swimwear for women and is very experienced in dealing with stretch fabrics. He has, I think, also made bras for his wife. Fun guy, with a mad selection of fabrics and machines!

This lady in Oz will take your ancient tatty old bra and make from it a perfect pattern, and send back that and a basic kit for you to make one yourself, or can use it to make for you: http://www.beautifulbras.com.au/book.htm#pattern She comes very well recommended in many places. It isn't a cheap service, but over the years could save you a lot.

One of the main things to remember when you set about bra making is that over 78% of women wear the wrong size bra! Many bra problems are solved by having a proper bra fitting session with a trained fitter who will find the correct size, shape, and make of bra for you. As with any other type of clothing, one maker's 32DD is not the same as another's! (I find in particular with this size that most makers are expecting it to be fitted to a teen/20's dolly with plastic inserts rather than a woman in her 50's with kids who is all natural!). When you are larger than expected (and all ready to wear clothing, whatever the size, from 00 to 8XL, is made for a B cup figure), the sums don't add up, and for greater comfort you need either to pay a lot more or learn to make your own. Bras and shoes are the two areas of clothing where I'm prepared to spend real money.

Remember also that RTW bras are made to last about six months if you wear them 2-3 times a week and hand wash them... The larger your size (cup OR band!) the more stress you put on it, and the shorter the lifespan of the bra will be. This information and the wrong size stats come to you courtesy of Charnos and the Designs on You Bra TV program of a few years back. I had a couple of the Bioform bras that came out of that program when I was a 38F/FF... I'm a bit smaller now! And I have been very tempted to make my own bras, but have not yet had the time to dedicate to acquiring the skill set.
Mad sewing witch!