I don't have a link to the thread, but one way of doing it is to simply start out doing a straight-leg push-up with your upper body higher, then gradually lower the level of your upper body until you are on the floor.
The progression would be as follows (surfaces are given for general height suggestions: start at a level that you can do, but where the last two reps or so are challenging. Move to the next level when you can do all reps in a set with no problem):
hands on wall
hands on kitchen counter
hands on chair seat or edge of bathtub
hands on high step
hands on step set at around 12"
from there, you just decrease the step height by 2" until you are just using the platform (4") then go to the floor.
By progressing this way (rather than trying to go from knee push-ups to straight-leg push-ups) you are strengthening the muscles used in the straight-leg push-up in a progressive way that transfers directly to the full-on push-up.
HTH!