If you have never walked a set distance before, start at home by marking out twelve feet on the floor (masking tape works well). If you want to be really accurate, put a little mark every three feet.
Now, place one heel on the tape at the very start of your twelve-foot line.
Take one big step forward with your other leg so that the heel lands on the three-foot marker.
Now continue walking so that your other heel lands on your six-foot marker, and continue like that until you have walked the full twelve feet. Keep walking back and forth down the line, making sure that your heels always land on the tape. Get a feel for how big that step is because you will be using those three-foot steps to make your measurements in the show ring.
Once you're consistently walking the line correctly at home, try the same thing in a sand ring at the barn. Place two rails twelve feet apart on the ground and walk between them until you are consistently getting four even steps. You should start by placing your heel up against the first rail and finish by stepping over the second rail with your heel just touching it. Measuring from heel to heel gives you a much more accurate measurement than using other parts of your foot. Chances are that you will need to make a slightly bigger effort when walking in sand because you will sink into it. Remember what that feels like; you will need to know how to walk in different types of footing.
Now you can try walking a line! Set up two jumps in a straight line, some multiple of twelve feet apart. The line below is 36 feet long.
Your average horse's stride in the jumper ring is twelve feet, which corresponds to four of your steps. Your take-off and landing will each take up about six feet for a combined total of twelve feet. This means that when you are faced with a 36-foot line, your horse should take two strides between the jumps.
In order to walk the line, you should stand at the center of the first jump with your heel directly underneath it. Now walk forward, using your three-foot stride, and count how many steps you get between the jumps, finishing with your heel directly underneath the second jump. Different methods of keeping track of the steps work for different riders. Some like to count how many steps they have taken and then do the math at the end of the line. I, personally, count with my fingers by sticking one finger out for every four steps that I take. At the end of the line, I fold one finger back down to account for the combined take-off and landing distance and the number of fingers that I have left sticking out corresponds to the number of strides that I should put in the line.
Always remember to take that take-off/landing distance away from your count. If you reach the end of the line and you have to put an extra step in to get your heel right under that second jump, the line will ride forward. If you reach the end of the line before finishing a twelve-foot set, the line will ride short.
You don't have to walk from the center of each jump for a straight line, but you do need to be consistent in where you walk from. Unless you plan on angling a line, you cannot count accurately by walking from the center of the first jump to the right side of the second jump. If you start walking on one end of the first jump, walk to that same spot on the second jump. For a bending line, walk from center to center (or whichever spot you plan on jumping from) and try to walk a line with the same degree of bend that you plan on using with your horse. Swinging out wide or jumping to one side will change the amount of ground that you need to cover.
This is something that you will need to practice a lot at home before going to a horse show because you won't be able to use tape markers or a measuring tape at the show!