Ok,
I assume you have that little black tool with all these threads on it, and a bolt that screws in the middle.
Undue that bolt in the middle and smear all the threads with oil. Dip it in oil if you have to, that way the theads get lube entirely in the grooves.
Screw in the large part of the tool with fingers only in the shaft where the large gear is, being very carefull not to cross-thread at the start. Work it in slowly back and forth as if ya trying to crack a combination lock, ever so carefull cos the threads on these thing are not the best and start thread, CAREFULLY.
Once you have the threads started, wind it in all the way as much as you can, wind it back out half way once, so the threads just work a bit and clear with that smear of oil, then wind it back in again finger tight, then nip it up with the spanner. No need to go too tight, just flick the spanner at the end so it tightens and ya feel it's firm and a bit more.
Wind in the small bolt in the middle of the tool, same thing with the thread process at the start, finger tight first, then tighten with the spanner that fits the small bolt. Get to firm and a bit more to.
Then, put a spanner on the large part of the black tool so ya hold down anti-clockwise, and screw in the small bolt with the other spanner clockwise, while holding the gear and stopping it from turning with the big spanner. Wind in the small bolt and you should notice the gear sliding out. It might make a crack sound at first, you'll know it ain't a broken crack sound, but the sound of the pully initially moving off the shaft. Then again it may not, it depends.
Keep winding in the small bolt till the large gear is off.
Make sense ?