Another exercise that I think is really good, is one which I use often myself.
Put your horse on a circle and ride a little shoulder-fore before you go into trot.
Because you’re riding your horse shoulder-fore, the inside hind leg has to come a little bit more under the mass.
So you activate the inside hind leg, which enables your horse to carry more.
And you can use that to go into a collected trot.
It might happen that your horse goes into canter because he’s less restricted, but that’s okay. Just bring your horse back into trot and try it again.
Remember that you’re actually trying to slightly stimulate the hind leg before going into trot.
So, to prevent your horse from going into canter or if you want to restrict your horse a little bit, you can ride your horse shoulder-fore along the wall or ride nose to wall leg yielding.