[header] How to make a tire swing [title] Find a suitable old, unwanted tire. [step] Make sure the tire is relatively clean and be sure it's still in good enough condition to not split under the weight of people. [substeps] The bigger the tire, the better, up to a point. 
While you want plenty of room for kids to sit in the tire, a really huge tire will be especially heavy and may weight too much for a standard tree branch. Use your good judgement about the perfect balance between size and weight for your specific branch.