I used a measuring tape to measure the width of the tire from the inside sidewall across the top to where it would meet the chain on the outside sidewall. For my tire, it was about 18.5 inches.
Now measure that same length on your chain and cut the chain to this length. Make sure to take into account the length of the quick links on each end, usually this means you can cut your chain about 2 links shorter. For our 18.5 inch sections, it was 11 links long plus one quick link on each end.
Next, you will need to decide how many cross sections you want. I decided on 8. Now you have to mark where each of the cross sections will attach to the long chains. Our long chains were 64 links long, which means the cross sections will be attached at every 8 links. (64 links divided by 8 cross sections= a cross section every 8 links).