The most important issue is safety at all time for you, the horse and your nearest environment.
Before beginning to teach your horse have a look on the "rules" below to avoid the following:

• Do not let the horse stand onto your feet.
  Needless to say that this is very painful and you risk a fracture.
  
• Never get into a position, where the reins are too far away being gripped quickly.
  
• Never leave the horse facing an open gate as it might want to walk on and go home on its own.
  It can easily damage itself, the equipment and cart and the fence posts.
  
• Never remove the bridle during a break or a rest.
  The horse thinks it is free now and almost certainly takes off.
  
• Never stand in front of a shaft pointing to you.
  In case the horse jumps forward unpredictably, you might suffer nasty and often deadly injuries.
  
• Never climb on a cart or vehicle via its wheel spokes.
  Very bad accidents happen, if the horse moves forward suddenly.
  
• Never drive sitting on a shaft or another vehicle piece which is not meant to be sit on.
  
• Never train two green horses together on a vehicle.
  Always use one older and experienced horse beside the green horse.
  
• Make sure the harness, traces or chains, shafts and the vehicle are in good order and well maintained.
  

The principle of horse training is to make the wrong things difficult and the right thing easy.
When handling and training a horse you need to apply three principles:

• your brain
• apply pressure to get something you want from the horse and do not forget to release, when getting the response.
  
• know exactly what you want from the horse: it is not enough that the horse moves somehow, you need to know exactly how he should move.
  

When you cue a horse for something, he should do it.
Do not give up too early, if the horse does not do what you want him to do.
This is then what you need to practice with him.
The horse needs to respect you at all times.
Therefore never let him eat from the rein or your hand.
Once you allow this, he will disrespect you, you will loose control over him and you will come easily into dangerous situations.
Do you worry your horse unknowingly? How do you approach it? How do you speak to him? Try not to scare him, give him time to figure out what you want him to do, be patient.
Make obvious what you want.
Your horse needs to have faith in you: stay calm and lead him.
A horse's fear is instinctive; they are evolutionarily seen still animals of prey.
Fearing is surviving.
Try to focus on what you want the horse to do and avoid focusing on the horse spooking.
Always stay consistent.
Teaching through leading.
A lead rope is a very worthy instrument for teaching a horse.
Always teach your horse the right "answer".
Use the pressure - release - method.
Pressure = asking to do something, release = horse can relax and gets time to understand what you wanted him to do.
This way he learns the "answer".
You need to know the answer beforehand to judge, if it is the right one.