There are a few ways to start a stitch. Here is a simple and clean method that works well regardless of the number of strands used :

Bring your needle through a hole from the back of the fabric while leaving a short tail. Send the needle through the hole diagonally opposed to form half of your first stitch.

Flip your fabric to see the back. As you push your needle through the next hole, make sure to catch the thread tail to secure it in place. Catch it a few more times as you go.