parts which are in original filament sensor for Prusa:

1x IR filament sensor
1x cable for filament sensor
1x steel ball 7 mm
2x magnet 10x6x2 mm
1x M2x8 mm
parts which you need to source elsewhere:

2x M3x12 screw with round head or M3x10 with hex screw head

Assembly manual:

remove support material from the print
insert steel ball
screw in lever with hole for magnet with M3x12 roundhead screw
screw in IR sensor with M2x8 screw
screw in second lever with M3x12 roundhead screw
put magnets in so they repel each other
test if machanism work with piece of the filament
put cover on

Extruder disassembly:
unload filament
unscrew extruder by this order → hold it so it will not fall down
loosen screw which holds PTFE tube (1)
remove screw which holds extruder together (2-4)
remove M3 locknut (5)
push out bearing (6)
remove tensioning screw (7)
remove brass fitting (8) with universal wrench from Prusa Mini kit (or with other tool)

Extruder assembly:
replace rear extruder part with new one
go backwards in disassembly steps
TIP: I recommend use 10 mm socket wrench for brass fitting, as it is much easier to allign it with the hole and screw it in. Be carefull not to bend short PTFE tube!

Last part is connect cable from filament sensor to Buddy board. Follow official manual for this.
Note: I do not know if official cable will be long enough, as I used custom made one.
Note #2: User Amperage confirmed in the comments that original cable is long enough.

I wired my non-original cable this way:
push PTFE tubing (for filament) in to the rear extruder part. It should hold in on its own by friction.

Warning!
There is issue with extruder colliding with sleeve for X-axis motor. There is not enough space between my version of rear extruder part and Z-axis profile, but I can do much about it. Solution which I´m using is pull-out sleeve from the profile and leave there just cables as you can see at pictures below.

I hope this will help you make this work. If you will have any question, do not hesitate to ask.