I will have to keep my eyes open for the mini Xmas moss...
You asked how I attach the moss.
I started off using little elastic bands. These work really well. Much easier, quicker and more adjustable than fishing line or wire.
But more recently (since the moss has grown thick and mat like, I usually just wedge it into the notch of a root. Or stick a finger through the mat and then ‘hang’ it over something like the end of a stick. It looks a bit silly for a day or two, but soon fills in and covers the ‘hole’ - and of course that stops the tank being cluttered up with elastic bands. :)
I must admit that I buy most of my plants from the UK online shop Aquarium Gardens. One of the many things I like about them is that all their plants are clearly marked with their difficulty and requirements. I started with the easier ones, but may explore further soon.
I have three types of moss, but all I know is that I keep in water and it grows. Heck, I keep it in a semi sealed container and spray it with water once a week and it grows, glued to some shells. I don't even know what kind of moss it is. It grows.