Asking the paces to become bigger takes a lot of balance from the horse. It is therefore of utmost importance that the horse is absolutely balanced, and travelling with engagement and softness in your hand and seat before asking for lengthening of the paces. It is so easy to scare a horse in a lengthen if you allow them to lose balance, which happens when they are not sitting over their hind legs.
If you are looking just to get your horse travelling forward with a soft jaw and start of a connection from the hind legs to your hand, don't expect results overnight.
Your seat is the key part of this equation. Kicking and squeezing will do nothing if your seat is not controlling the tempo. The seat acts as the dictator, over tempo, pace and direction. The leg and rein aids merely support the seat.
To have an effective seat, you must have a strong core, and to have a strong core, you need to do some work off the horse.
I do core work every day, and run most days of the week. Pilates is also very useful for developing core strength.
The importance of having a strong core, is because your core is essentially holding your horse together. You should feel that by engaging your core, your horse brings his hind legs through and the wither will lift.
To test if your seat is effective, you should be able to stop, change speed and change direction with no, or very little input from the rein and leg aids.
If you can master your seat, you will be able to master true collection.
When you're seat is effective and correct, you will be able to hold the horse's tempo, and put your leg on to ask for a bigger and move active stride. This in turn will begin to develop contact in the bridle, and the jaw and poll will begin to soften.
Definitely not a matter of 'kick a bit and pull a bit'