See http://gamedoctor-sf6.blogspot.com/

Download one of the bin datas of the UCon64 website:
uCON64 bin data for Windows

Then you have to download a GUI:
uCON64 GUI:uf-FOX

Additional you can download a Super Nes dat file, which makes the options easier to use for you:
uCON64 Super Nintendo dat file

Then open the front end’s binary. Then frontend (menu item) -> Option and put in ucon64’s full path.

Then under Conversion section (of SNES tab), choose ROM to GD3, then click convert and select your .sfc file (I haven’t gotten smc to work)

You may have to click “Abort Ucon64” since it seems to get stuck 100% of the time when it’s done.  Just look for the output file (by default, in the frontend’s directory)

Make sure the output file has .058 (or .078?) suffix.

If the file is greater than 1 floppy can hold, you can, from command line (since frontend UI seems broken):

ucon64 -s -ssize=1.4 <filename>

That will automatically output files with the right file extension.