Sorry Chaps , been away and ill so missed this
I bought a halti-harness for Molo a year or so ago - started with a medium, thought it was too big so bought a small - that was definitely too small, so ended up e-mailing the Company of Animals asking for advice and had a brilliant e-mail back from one of the staff who also has a cocker spaniel and sent me a lovely picture of her dog modelling one correctly fitted
The central ring is designed to be stitched into position once fitted otherwise it slips and the harness will not work. This black strap does not need to be tight against the body - but the ring must be central and clipped to the collar. The tummy strap can be adjusted using the top adjusters and underneath strap so that the padded bits fit underneath the front legs and it is snug (two finger tightness like a collar) - excess strap can be cut off and the end resealed with a naked flame.
I began by using a double ended lead, but soon had no need too - and Molo has always walked wonderfully on the halti-harness with an ordinary lead clipped to the front ring despite pulling like a train on everything else; we are slowly graduating to lead walking on a normal collar now though
They are a real faff to fit at first, but once done, I am really pleased with the results - so they are worth persevering with if you have a dog that pulls