I removed one orchid from a 2.5" pot and the entire root ball came out so I just attached the entire thing to a mount. It did very well and I'll cut the root ball off later when it's grown some. On some orchids the media all fell off so I just mounted them bare root and they attached well also. I don't cut off any dead roots at all. They just dry up on their own an fall off. I want to put as little stress on the plant as possible. I've put sphagnum on some and left it off on others and they've all done well so far. But I do live in a hot, humid area. I water every day until some good roots have grown out.
---------- Post added at 05:41 PM ---------- Previous post was at 05:11 PM ----------
I'm going to post a few pictures because I mounted several small catts over the last few months. I attach them with lots of different things but this one is attached with velcro tape that I found and Home Depot in the garden section with the trellises. A little sphagnum stayed on the roots so I just left it.
The root ball and all the media stayed together when I pulled this one out of a 2.5" pot. I strapped the entire root ball to this tree fern and within 6 weeks it was attached enough to remove the ties. After it grows more, I'll cut off the old root ball. Some of the roots have grown right through the middle of the tree fern.
I put a little sphagnum behind this one to hold the new growth a little bit away from the cork to give it room to grow. I mounted this one in January and took the ties off after two months.
I attached this one bare root with no sphagnum or medium. It attached very quickly but it took three months to develop roots on leading pseudobulb. When that area finally attached I removed the ties. By the way, it's not necessary to remove the ties at all. Also I think I may be mounting my orchids too low on the mounts. There's very little room for the roots.