What do you mean with "make a custom WFC"?

There's AltWFC which is another custom server for DS and Wii games, which you can find here: github.com/barronwaffles/dwc_network_server_emulator
Though it doesn't have as many features and isn't as secure as Wiimmfi.

As for completely making your own from scratch, writing your own servers, it's not really possible to write a tutorial to do that ...