Hey PL as has been said, collection takes time especailly if you want to do it properly and not just make your pony put it's head down.
In the German Training Scale, Collection is the last step. You need to be able to do everything below in order to achieve true collection.
Have a look at this to see what I mean GTS.jpg
There are some good books you can read to help you on the path to achieve this, but you can't just learn from a book. You do need a trainer to help.
Principles of Riding by the German National Equestrian Federation is a really good book which has helped me a lot.
ArtToRide has a really good youtube channel as well ArttoRide's channel - YouTube