This is the easiest guide to DIY a Game Boy flash cartrige, you only need to solder 3 or 4 wires and the adapter board to an old cartridge.
Maybe are more cartridges compatible with this adapter board, you only need to check the footprint of cartridge ROM is the same as the adapter board:
This adapter is comptible only with 32 pin SOP ROMs
First one is desolder the read-only ROM on cartridge, do it carefully to avoid damage on cartridge PCB. Normally the ROM is marked as U1 2M/4M/8M -MROM, below the backup battery.
The next step is solder the adapter in place of ROM, I recommend cleaning the remaining solder (with some solder flux and copper wire) before solder the adapter.
And the final step is solder the 3 or 4 wires of signals: RESET (/RST on MBC5), RW (/WR on MBC5), A20 and A21 (*You only need to solder address 21 [A21] if you have a 4MB ROM adapter). You can solder the wires directly on the MBC5 or on a via/trace that carry any of these signals.
The first time when you program your cartridge, you will need first erase the memory clicking the Erase Flash button.
After this you can program the cartridge like others: MBC AUTO… FLASH/ROM size… RAM size…