I just use the standard
Kester 44 Rosin Core Solder 60/40 for all my work. I would not spend extra for any
Audiophile labeled solder.
I just removed the cable from the cup and widened the hole with a drill bit by hand as the plastic is very soft. The 3.5mm socket I used for them was salvaged from a headphone extension cable that I did not like. Just cut the plastic away and unsolder the wires for a nice socket you can repurpose, check the size of the socket before you enlarge the hole in the cup just to be sure.
There is not a lot of room behind the driver so you sill have to hot glue or epoxy the back side of the socket to keep it in place. Also be careful of what size plugs you want to use. The YARBO above fit perfectly, but the big Pailic plugs with the white band at the top will not as their base is too big around.
I don't know how much soldering experience you have but this is not that hard of a project. The Ultrasone drivers are not like the beyerdynamic ones so you don't have to worry about breaking a voice coil wire.
Have fun, ask any questions if you are stuck and be sure to post some pictures.