Timing and circumstance are key.
Weaning varies from litter to litter. I would watch the mom's cues and the kittens actions.
As early as 3 weeks you can begin feeding the mom her wet feed in the cage with the kittens, see if they are interested, but dont worry about encouraging them, a lot of kittens wont wean until they are 6 weeks... although I think the average is 4 weeks (remember... they will still nurse for several weeks after the introduction to solid food).
Kittens in large litters tend to wean themselves earlier as there is less competition and more room in front of a dish than with fighting for the perfect nipple. Singletons tend to nurse longer as there is no rivalry and also it is less stressing on the mom's body.
Mother's will naturally begin to spend more and more time away from their kittens and will begin the weaning process for you... watch for this.
When I wean our fosters I mix dry kitten food with water or KMR until it is mush. Always watch them in the beginning to be sure no one chokes. And be prepared for them to do nothing but sniff it or walk through it... If they are older, place some of the slop on their lips and they will lick it off, once they get a taste of it, they will be more likely to try it on their own.
Good luck. And take pics!