Tie a knot by the base of the seam where the thread is loose.
Trim the thread down to the seam with a pair of scissors.
Inspect the tear to determine how to proceed. If only the thread tore, you can simply replace it through the same holes. If the leather tore, you must create new seams.
Double-thread a needle for sewing new seams.
Put the threaded needle through the seam holes, if they're intact. If not, pull the leather closer together to create a new seam. You can use a leather punch to create new holes for threading and creating the seam.
Tie the thread once you reach the end of the seam. Use scissors to trim the thread by the seam.