In what? a real boat, toy model?
If it's a real yacht....You cant, unless you've got a sailmakers sewing machine....Unless the tear is small and you can make a patch which can be handsewn over the tear.
Go to a chandler, or look in Yellow Pages..Pay the money...Be safe it could mean your life, if out in the Channel
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix sports equipment.