There are four ways of starting a waiting list.
1. For a known and well established breeder a waiting list will usually start straight after the last litter has gone and people will stay on that waiting list for a good year to make sure that they are getting a good dog, from a good breeder.
2. Once you have decided on the stud and the bitch to be mated, then you can advertise a pre-mating between the two at which you will get interested parties wanting to be on the list.
3. Or you may advertise after the bitch has been mated and pregnancy confirmed.
4. Lastly, once the pups are born they can be advertised and a list compiled.
5. Not do one at all, and just advertise and hope for the best.

(As you can see not in favour of that one!)
1, 2 and 3 obviously give you much more time to sift through and get to know potential owners.
4, gives you much less opportunity although you can still get lots of good potential owners. You need to be a good judge of character to wait until the end.
I much prefer 1 or 2 as it takes away a lot of the worry about who will be a good owner for your pups and leaves you to concentrate on the whelping and raising of puppies. It also gives you time to get all the paperwork ready and named, to have names, addresses, tel no's etc. already on file and gives you time to bond really well.