A.How to relocate the analogue stick and hold it in place
Cut it out, use some bonding putty to reposition it, and hot glue the analogue stick's base to the pcb after bending the legs outwards so it sits flat.
B.How to fit a 3ds slider as a dpad
3ds slider? Well, you can look up a pinout/trace schematic of the controller's pcb and solder wires to the D-pad's traces to add your own buttons to do the same functions.
C.How to wire it all up
With a soldering iron, solder, and some insulated wire (flux might make it easier also)