How do I use Velcro to fix fitted sheets?

Google it.

I find Velcro to be a somewhat destructive force in the washing machine and dryer. It can be abrasive possibly damaging to fabrics and eventually collects fibers and lint in the loops. Sheets demand frequent laundry care. Imho- I would not suggest it —-I would suggest finding another way to make your fitted sheets fit better.

Sometimes it takes adding an accessory like fitted sheet suspenders or replacing your elastic to make it tighter.

Mattress have gotten very tall over the years and generous deep pocket sheets seem to be more common than the fitted sheets of years ago.

Alternatively another option would be to sew more elastic on your sheets to alter to make a tighter fit. Heres some helpful links that might give you more ideas and details. I hope this helps.

https://www.ehow.com/how_5549201_repair-stretchedout-fitted-sheets.html

https://www.linenslimited.com/blog/this-is-why-your-bed-sheets-keep-coming-off-4409/

I wouldn't go with velcro. Either straps or this thing I recently found. I'm thinking of trying to make one.

https://www.thebetterbedder.com/

That project was about making a cornice curtain over a toddlers bed.

I use large safety pins, through the corded edge of the mattress.