The military uses equipment that lets a service member carry everything he owns with him. Besides a huge ruck sack and multiple pockets on the uniform, a basic item issued to every soldier, sailor, airman or Marine is the molle vest (Modular Lightweight Load-carrying Equipment). This vest is made of camouflage nylon with mesh siding, and it has row upon row of tough, stitched nylon bands all the way around it where additional equipment can be connected. There are a few items that already come with the vest, but most service members choose to attach other necessities as well, such as small flashlights and knives.
Check to see if your knife sheath has a belt loop on it. If not, you can cut a few small holes along the length of it. This is recommended so you can attach the sheath securely; you don't want it to fall off in woods or in the middle of the desert.
Invest in a bulk pack of small- to medium-sized zip ties. These can be purchased from almost anywhere, from electronics stores to auto shops to several companies on the Internet, and a 100-pack usually costs under £6. You can use them to attach other items to your molle vest later.
Using one zip tie, attach your knife sheath upside down to the thick nylon bands running around the front of the molle vest, at about shoulder level. Put the vest on and place your knife in the sheath.
Reach up and grab the knife handle, which is upside down so you can pull it out easily. You need to have a snap on your sheath holding the knife until you grab it. If you don't, this is not a knife sheath you want attached to your vest. Even if it were not upside down, an unsecured knife will fall out of your vest right away once you're in the field and moving around.
Once you've checked that the knife is in a good position and you can reach it easily, secure it with more zip ties.
Alternatively, you can use 550 cord to attach the sheath instead of zip ties. Cut four six-inch lengths and take out the nylon strings inside the 550 cord. Burn the ends of the cord until they melt slightly to prevent them from fraying, and tie them with a double bowline knot.