The BBC Master Compact (MC) computer has a separate unit (Disk Drive & Monitor Stand) that houses the power supply and 3½" floppy disk drive. This unit is not really necessary to use the Master Compact as any 5V 2A power supply with 2.1mm plug could be used (i.e. even a USB charger), and a Gotek can replace the floppy drive (apparently not much software came on 3½" disks for the BBC).

The MC uses a DB25 socket for the floppy interface. If you have a MC but it didn't come with the disk unit then you can make up your own floppy cable by either using a male DB25 plug with crimp connector and 34-way ribbon cable ... or build this simple board that uses a DB25 plug with solder cup connectors and re-use an existing 34-way floppy cable.

Some people prefer to make up a cable using the crimp connector, some people prefer to make up a simple board. Either way, the pin out is pretty simple - DB25 on left, floppy ribbon cable on right:

 1...INDEX.....8
 2.../DS0.....10
 3.../DS1.....12
 4.../DS2.....14
 5.../MOTOR...16
 6.../DIR.....18
 7.../SEEK....20
 8.../DATA....22
 9.../ENABLE..24
10.../TRK0....26
11.../WRPROT..28
12.../READ....30
13.../SIDE....32

(see also my Github page)

If you want to use a DB25 with crimp connector then do the following:

[1] Pins 14 to 25 on the DB25 and the odd-numbered pins on the ribbon cable are ground and thus should connect,

[2] Pin 8 on the ribbon cable connects to pin 1 on the DB25 - discard the first six ribbon conductors (1-6) and the last two (33 & 34)

If you want to build my board, just get a generic 2x17 pin header and male DB25 plug, solder them to the board, then connect with a standard 34-way IDC ribbon cable to the Gotek.