Not an easy, straight forward question but I will try to make it short. I got my instructers certification with CHA 8 years ago and have taught lots of beginners. Hopefully this will be helpful.
First make sure they are using a good saddle that sits level on the horse and has stirrups that allows them to get their legs underneath them. Cheap saddles that don't fit the horse or rider well tend to put you in a "chair seat" which makes it nearly impossible to post or 2-point even if you do know how.
Then have them stand up in their stirrups while their horse is standing still. Don't let them lock their knees straight, just stand up so they clear the saddle by 2".....with their heels down. Adjust the length of the stirrups if needed. If you take your feet out of the stirrups the bottom of it should be near your ankle bone. Have them keep standing and sitting until they find a spot where they can balance without using their hands, they should always sit gently back on the horse...don't just plop. Make sure they keep their legs under them as they sit and not move them forward which commonly happens. To help illustrate the point I have my riders try it the wrong way too, do an exagerated (sp?) chair seat and have them try to stand so they can see how hard that is, then have them move their legs way back and try it again. Doing it the wrong way helps them feel the right way sometimes.
So now they have an idea of balance and how to get their legs in the proper spot underneath them. Have them try it at a walk and eventually a trot. Being on a lunge helps because they don't have to worry about steering and are less likely to catch the horse in the mouth....the reins should never be used for balance! Once they can stand and find their balance for at least 10 seconds and not plop back into the saddle you can introduce posting.
At the stop or walk call out "up.......down.........up..........down" allowing them to hold each beat for 1-2 seconds but not as fast as the normal rhythm yet. Explain the normal posting rhythm is much faster and remind them at this point that posting is not about how high you can stand up, you just want your seat to clear the saddle and keep your heels down.
Posting at the trot can now be tried, lunge line is great, a slower calm horse is even better. I usually have them start posting at the walk about 5 seconds before I ask the horse to trot on the lunge, call out "up..down..up...down" in time to the horse, I only do this for about 30 seconds or less then we walk and talk about what happend, usually the first time they post too slow or they post one beat and sit two beats. This is really common in the learning process, keep doing short trots and taking breaks to talk about it. Praise your riders like crazy when they finally get it even if only for a few strides, be very calm and patient through this. If done right, it can easily be taught in one lesson to anyone.
Good luck with them, pony club is a great program too and should help improve their riding.