Your horse needs to be able to eat from your hand or a bucket and to work on a line (dragging or tethered, your preference) if you don't have a small space to start training.
To teach eating from your hand, start with a bucket on the ground with some yummy food inside. Next, invert the bucket and place some treats on that. When the horse is eating successfully at that level, fill your hand with some treats and place your flat palm face up on top of the bucket. Over several training sessions, gradually raise your hand over several sessions until the horse is eating comfortably at your waist level. The clicker is not needed for these sessions.
Also, you need access to clean floor/ground space with no food available freely and no distractions, especially important in the early learning stages. The corner of quiet barn works great! For an outside area, try to work against a wall to limit visual distractions and wind.
If your horse mugs you for treats, use a barrier such as stall door or paddock gate to keep between you and him or tie him on a short line and stand out of reach until he learns food Zen. Keep yourself safe at all times.
Ideally, you want to work your horse off lead if it is safe for you to do so, as it simplifies management of lead ropes, tethering etc for both you and the horse and your horse will learn that being near you is far more fun and exciting than anything else he could do. It is also makes it easier to transition to training from his back.
Start with your horse by himself so he can focus on you and not be interupted by his buddies and you are not being distracted by them as well.
Keep them behind a fence, rope or tie them to a post out of distraction range.
In this video, the trainer would have done better to tether the older horse to a post out of reach of her and the learning horse. She does observe that the older horse is interfering with the new horse's learning, but chooses not to change the situation. This will slow the learning.
How many different distractions can you count that interfere with the horse learning his new task? (Start watching at 0:55 to save you viewing time.)
Video from desertduty's channel on youtube.com