Over time, the velcro that secures a baseball cap can collect dust and lint that renders itself useless and unable to bond. Instead of discarding the cap, you can a replace the worn out velcro by sewing a new piece of velcro onto the strap. For this fast fix, you will need to know how to sew using simple stitches.
Use a running stitch to secure the velcro to the strap of the cap.
To do a running stitch: bring the needle up through the cap side of the strap and then the velcro until the knot at the end of the thread hits the fabric.
At this point, bring the needle back down through the velcro a few millimeters from where you started, following the direction you're trying to sew.
Keep bringing the needle up and down through the strap and velcro, moving the same distance each time. Repeat this until you have sewn around the entire piece of velcro to secure it.