Personally I would only use a halti in combination with a harness and double ended lead so you have a point of contact on the harness and the other on the halti. You can really set up a lot of tension on the neck if you only attach to the halti, esp if the dog is a puller as you can end up pulling the head round a lot which puts lots of strain on it.
Also with 2 points of contact you can have the headcollar a lot looser so it's more comfortable, as you don't have to worry about the dog getting out of the halti because you have a harness on as well.
Prepare the dog as you would for a muzzle, eg lots of ttouches around the muzzle, a calming band or face wrap, lots of positive reinforcement and then slip the halti on briefly. Most dogs will accept it as long as you put in the preparatory work first.