how to fasten sport band?

First make sure that your band is inserted properly into the watch...the side with the size markings and indentations go on the inside, with the smooth, flat side of the band on the outside. If you put the bands on inside out, they won't fasten properly.

I lay my watch on my wrist, then turn my wrist over so that the watch is on my bed, and the ends of the band are facing up. Lay the pin side over the inside of my wrist, hold it with my thumb, and use the fingers of the same hand to grab the other half of the band and pop it over the pin. Then I can pick up my wrist and tuck the end of the band into the slot while holding a finger over the pin so I don't pull the band off the pin while tucking.

It sounds complicated, but I've done it this way hundreds and hundreds of times since the original Apple Watch, and it takes just a few seconds. I'm clumsy, so if I attempted to do this while not over the bed, I'd surely drop my watch.

Hope this article may help

Apple Watch Band Tips

https://support.apple.com/guide/watch/remove-change-and-fasten-apple-watch-bands-apd3cc13d48f/watchos

May I suggest you take a look at the following bands, they are much easier to put on and adjust and I think you will find them more secure as well.

Sport Loop Bands

Solo Loop

Braided Sport Loop (my new favorite, once I got the size right)