A Halti is a type of head collar for dogs. There are a variety of brands, each with a slightly different fit and style, but that all work basically the same way. A head collar works much like a harness on horse. It lets you guide the dog, redirects the dog's attention to you without a harsh correction, and discourages the dog from pulling or being overly excited. It consists of one strap around the neck and one around the nose. You can make one at home in a few simple steps. Before you use it you'll want to learn more about the adjustment process and proper training.
Take a slip lead that is the appropriate size for your dog and form a large loop as though you are going to put it around the dog's head. Hold the lead with the loop in front of you and the long part of the leash hanging down.
Twist the loop in the middle to create a figure 8.
Take the piece of nylon and wrap it vertically around the X at the middle of the figure 8.
Sew the piece of nylon closed at the ends. Make sure you've left it just loose enough to let you slide the loops of the figure 8 to adjust their size.
Slide the loop on the leash end of the figure 8 around the dog's neck and the smaller loop around the dog's nose with the middle of the figure 8 situated below the chin. Use like a regular head collar.