Take a pice of thread and needle, tie a knot in the end of the thread and thread it through the needle. Push the needle in from the back of the hole (so that the knot is on the inside) and sort of stitch it up like you would imagine bringing two sides together. Like in and out and in again - brining both sides togehter to make a seam.
Ideally you'll take th over off whatever it is and stitch it up from the inside out so that the thread isn't visible fromm the outisde but this issn't always possible.
THis might help:
https://www.youtube.com/watch?v=jLnPsryAQTM