Just to reiterate what others have touched on....
You are getting slightly confused between using leaves and stems from flowered plants and those that are still vegging.
When people talk of making hash and oil from leaf and stems, they are referring to that which is chopped off a plant that has finished flowering. When you crop a plant, you have all of the bud, but you also chop off a load of the larger leaves and many bigger ones poking out of the buds. Along with this you chop out many of the main stalks and stems. This way you get a load of nice bud and all of the 'waste' you use for making into hash and/or oil.
This is very different from using leaves and stalks from vegging or part flowering plants as the resin glands have either not formed yet or those that have, have not matured enough to contain many (if any) psychoactive ingredients. Sorry mate.

It's not until the buds are fully mature and ready to be cropped themselves that the surrounding leaf matter and stalks will be worth using for hash and oil making.
It's also worth baring in mind that not all plants and strains take the same amount of time to mature as well. Depending on what strain it is that you are growing, they could take anywhere between 7-12 weeks on average for the kind of plants that are generally grown indoors in this country (maybe even double that if it is a sativa dominant plant you are growing). So don't work to a rigid timescale, but rather react to your plants and let them dictate what you do. There is no point putting lots of time and effort in over 12 weeks getting a plant through veg and to full flower and then chopping it 2 weeks early when it is not fully mature. The difference 2 weeks can make if the plant needs it is phenominal, believe me.
There are various ways to tell if your plant is fully mature and at its peak, ready to be cropped, trimmed, dried and the trim made into hash. The best and most reliable is to use a magnifying glass or lupe. Look at the capitate resin glands themselves under magnification (the stalkied trichomes that look like tall mushrooms - stalks with balls on the end). When over half of these have started turning milky coloured instead of totally clear and see through, but not amber coloured (I am talking generally here for most strains rather than the odd few that are better when they start turning amber), then they are ready to crop. This usually coinsides with the changing colour of the hairs on the buds, but not always. You can use this as a rough guide though - when two thirds of the hairs have turned from white to orange and shrivelled up a bit, the plants are round about ready to crop if you fine tune by looking at the resin glands themselves.
I hope that this helps and clears up a few things, and that I haven't confused you even more!
Ask away if you want me to explain anything in more detail or clarify anything.
Good luck with the grow and happy hash making if you go down that route.

Edit: To tidy up spelling/typos a bit!