You need to ride lots of transitions in and in between gaites, to balance your horse and make him stronger in order for him to be able to bear more weight on his hindlegs, which is what collection is about :-)
Think up not back. You want the horse to have the same amount of energy (not slower) but to be covering less ground.
It’s all about the balance between leg, hand and seat. To collect you slightly increase the hand and seat aids, while maintaining good leg contact. Also, start with small amounts of collection. Like when learned to extend, you start with a lengthen and then progress.
The best way to begin asking your horse to collect is ensuring they are able to push from behind. Achieving this is best by doing a lot of transitions and doing lengthening and collecting of the walk, trot and canter
I'm probably like a broken record on this now, but Scales of Training! Collection is the end goal. It's not something that can be achieved overnight even or in a week. It does not involve hauling the horses head in, and the term 'on the bit' refers to the final symptom of collection, when all over things are in place.
The best places to look for tips are honestly YouTube and Twitter - the pros will break down their own horse tips in an easy to understand way. What I’ve seen boils down to :
1) have your horse moving forward at a steady clip
2) be sure to have active hands once your horse is forward enough and into his/her bit