What species are these cactus? Do they look okay or are they dying. I put them by the window.
I am just very worried about them. The tinest cactus has turned completely black.
Certified GKH Gardening Expert
It will be very hard to tell, exactly, what that is. When it is that young it will tend to look very similar to many others.
In order for them to root, you will need to find out which way is the bottom of the cactus. This will usually be what looks like a "cut" end. The top will be an endless set of spine rings getting smaller and smaller towards the center.
You will need to expose that bottom end to sand, or something that it can have direct contact with, until it roots. Then you can plant it as you would any other cactus. As it is, the spines will prevent the bottom from touching the potting media, so you will have to push it into something soft, and sand would do this.