Female turtles lay unfertilized eggs without ever having seen a male turtle. It is like chicken that lay eggs without "a chick in it".
If they do not have chance to lay these "empty" eggs, they will become ill and eventually die.
As far I know, the turtle wants to make the decision of when and where to lay her eggs herself. She should be able to reach the sand location at her own volition. You must not move her there. This could be difficult, but here you can see some examples how it could work. (Site is in German language, but all links in the pointed list show pictured solutions for sun and sand places. For example "Eiablageplatz" means "egg laying place")
(My own yellow bellied slider absolutely won't accept it if I catch it from the water. It is like a big predatory bird would take him as dinner...)
Another tricky thing is the composition of the "sand". For example if it is dry, the turtle can't dig a hole, because the sand runs back into the pit. To mix sand with soil is a solution for this. You can test the mixture, if you try to dig with a spoon in it: it should be easy to dig, but the walls of the hole should be stable.
The location of this place is essential, too. Your turtle is a smart one. She "feels" the water level and always chooses a place above that level, so her eggs won't become wet.
Another point, because you write "one of my turtles":
Your turtles, like mine are " loners" most of the time. This means they stay alone until they want to match and make little turtles. Because of that it will be tricky to keep them all together. If they are grown you need a lot of place if you want to keep all together. For example everyone needs a separate sun place (basking spot) because the biggest turtle will chase the smaller one off. But this biggest turtle wants to stay at the sun place as long as possible... so the smaller in fact can't use the sun place ever.
To transport my turtle to the vet I use an old towel. I water it and catch the turtle with one hand from water. In the other hand I hold the towel and wrap it around turtle. Then I put all together in a big bin and open the towel. The towel stays in bin as padding and water reservoir.