When you start a young (or inexperienced) horse under saddle you always start with a "direct rein". That means if you want the horse to turn left you "open" (the left rein. The word open here means take that rein and position it more towards the riders knee, NOT backwards.
Now if they know nothing they will bend their neck left (but not follow the neck with the body). That's why good trainers start the horse in long reins - it's called ground driving. That means that the "soon to be rider" stands behind the horse with a long rein in each hand. In this case when they open the left rein they "lay" the whip on the right side of the horses body and "push" the horse left. (do NOT hit the horse with the whip and the horse should NOT be afraid of the whip, just respectful.
Being inexperienced the horse MIGHT take only 1 step left. The handler should provide positive verbal feedback (Good boy/girl) and continue lesson. Eventually horse should trun left and right based on open rein reinforced with whip on opposite side of open rein.
So the rider should start with open rein and instead of using whip on opposite side instead rider use leg on opposite side to move horses body towards "open" rein. That may mean pushing/bumping horse with riders leg on shoulder/at girth. One step is good boy/girl.
Soon horse direct reins (using legs and open rein). When you have that down you can introduce neck reining. This time when you bend the horse left by opening the left rein you "push" the horse to the left using the right rein on the horses neck. Immediately after applying outside rein to the neck rider should apply outside leg to horses shoulder/girth area. It may also help to SLIGHTLY weight inside stirrup (left stirrup in this case) and look inside (left).
At any time if something does NOT work don't get upset with the horse - they're trying to learn a new language and like some people, some horses don't learn as quickly as others. Just go back to the last step that did work until they're consistently doing what you want, then re-ask by introucing new stuff (indirect rein on neck) BEFORE reinforcing with old cues (outside leg).
Dressage is for Trainers!