Cut one 12-inch length of ribbon with the scissors for each point where you wish to secure the mattress.
Thread the end of one ribbon length through the eye of the yarn needle.
Insert the yarn needle through the mattress at a corner where you wish to secure it to the porch swing. Push the needle back out again after catching about 2 inches of mattress fabric between the entry and exit points of the needle. Pull the needle through to the halfway point of the ribbon.
Loop the ribbon around a slat or chain at the corner of the porch swing to fasten the mattress to the swing. Tie the ends of the ribbon in a tight double knot.
Repeat the same process to tie the mattress to several points on the porch swing.