Take the excess thread and loop it to make a loose knot.
Slowly tighten the knot, keeping it close to the fabric. Place your finger on the thread to hold it to the fabric as you finish pulling it tight.
It may take a few tries, but once you get the hang of it you will have a nice knot at the end of your string! Do this a few times to make sure it is snug.
I am a complete beginner when it comes to sewing. The first time I did a project requiring hand sewing, I had to look up how to tie a knot to start sewing and how to tie a knot to end the sewing. It seems like a very simple thing to know, but there are a lot of people who don't know how. Here is a very easy way to help you tie a knot at the end of your string before you start sewing and a simple way to tie off the thread when you are all finished!
Starting With A Knot
Ending With A Knot