Some hotels leave free sewing kits in the room for you. Take 'em home for times when u might need them like now :) Or ask your mom if she has any lying around the house.
You need elastic band. Look for clothing (you don't need any more) with elastics that you can use. I'm using my (old, but clean) panties.
Disengage the elastic from the fabric by cutting up the thread. It's tricky but you can do it.
Cut the length that you think you might need. Mine was easy because much of the elastic was on the side of the panties already.
Put your hand inside to feel how much you would need to tighten up.
Most hats have this ribbon band on the rim inside.
Choose the thread color that matches closest to your hat color.
Put the thread through the needle's eyelet (hole). Tie a knot at the end.
Fold the ends of the elastic.
Sew the ends to the ribbon.
Make sure the thread is secured. Make a knot when you are done to keep the thread from coming loose.
Do the other end. Cut and adjust the elastic as needed.
Should look something like this.
Try it on to see how it feels. If it's still loose...
Make it tighter by folding the elastic in the middle part like this and secure with thread.
Keep adjusting it until it feels snug :)