Summarize:

Traditionally, there are 25 strands with 40 paper cranes each. You can divide the 1000 paper cranes any way that you wish, depending on how long you want each strand to be. With 40 cranes on a strand, use 1 metre (3.3ft) of thread. If you are only putting 20 cranes per strand, use about 0.5 metres (1.6ft) of string. If you want 10 strands of 100 cranes, you will need about 2.5 metres (8.2ft) of string or wire. Usually, each strand is about 1 metre (3.3ft) long. The length depends on how much space you want between each crane. The more space you want to leave, the longer the length of string you will need to use. Also, remember to leave extra string so that you can hang the strand when it is completed. If you want a much longer strand that will hang almost to the floor, use 2 metres (6.6ft) or more of the thread. You can make each strand as small as you like, but keep in mind that any string shorter than 0.5 metres (1.6ft) will not hold many cranes. Line up 40 paper cranes (or the number of cranes you are using per strand) and measure the thread or wire against them. This way you will ensure that there is enough thread or wire to string all the cranes. Place the end of the string or wire through the eye of the needle so that at least 6 inches (15cm) have passed through the eye. You will use this extra string to hang the strand. If you are using string, it may be easier if you slightly dampen the end before threading the needle. To prevent the bead from falling off the string, tie a loose knot at the end of the string that is furthest away from the needle. Then pull the needle and thread through the bead, until it is at the end of your string. The crane is now on the string; simply slide it down so that it is next to the bead. Pull the needle and thread (or wire) through the hole in the spacer bead. Then slide the spacer bead so that it is next to the crane. Spacer beads are not necessary, but many people find them helpful to prevent the cranes from bunching. If you are using 40 paper cranes per strand, you will repeat these steps 40 times until each crane is on the string.
Decide how many strands you want to make. Cut a long length of thread according to your desired strand length. Thread your sewing needle. Tie a bead at the bottom of the thread. Push the needle through the middle of the body of the crane up to the top. Add another bead to the string if you're using spacers. Repeat the threading process for each paper crane.