This was the simplest DIY I have ever done! I came across this while preparing for another one that I will share with you guys later. By using an old bra strap (the patterned adjustable shoulder part) I sewed together about one shoulder and a half to make this easy non slip headband. It took me around five minutes to complete including measuring it to my head but be careful, if pinning it to try for size, not to prick yourself! However, this DIY is only good for those bra straps that are stretchy and best when it has a non slip feature underneath (as seen in the pictures above).
Hope you like it!