I'm afraid there's no magic button you can press and.....bingo!...extended trot! There's an awful lot of work to be done before that happens. I suggest you get a good trainer. Apologies if my answer is less than helpful, but your question comes across as rather..er...basic(?) perhaps if you told us more about your horse and what your doing with it so far.
You need to get a proper correct medium trot first. Sorry if you're already doing that and I'm misreading, but extended is just "more" once you get a really correct medium.
Have you got a true working trot from behind with a good elastic contact and good movement over his/her back... once you have this then it is just a case of lengthening the horses shape a touch to get a lengthned stride (not faster) and then you go on from there til its bigger and bigger and bigger... it takes a lot of core strength from the horse so cant be acheived in a short while...
Extension is rooted in collection, it matters not whether the horse is riding school or not; develop connection, then collection, then the extension (well, generally lengthened strides to begin with, then medium trot) is there ready to access. Not to be done regularly even once established as it's very hard on the horse.