Should you half halt before asking for canter?

Yes. Half halts prepare and balance the horse for the transition, so using one gives your horse the best chance of responding to your canter aid quickly and staying balanced through it, giving you a good canter afterwards.

That's one way; even a "half of a half" halt cues your horse to expect a transition. Your best canter transitions will come from an energetic walk or trot; not a fast, or strung-out trot, but one that's a little slower, but with some energy.

(I may not be expressing myself well, here.)

I go from sitting trot then sit deep into the saddle amd ask for canter.