Not quite a foolproof tut on using goldleaf, but, I'll try help as I can.
1) Ensure you have libusbk drivers installed for the switch. You need to install them twice, once for RCM mode and once for OS mode. To do this, ensure USB is plugged in and use zadig.
2) Load up goldleaf using which ever method you use (I personally use the R+Album)
3) You should now open up the same version of Goldtree that you're using Goldleaf. So 070 with 070 if you're using latest version.
4) Go to explore contents, via PC (USB) (or along those lines. Switch isn't to hand)
5a) Should now be able to view the pc from your switch. Browse to the NSP you want to install.
5b) You can also open up a prompt within switch goldleaf to open an NSP via your pc. AGain, can't remember command but it's pretty explanatory...
Anything to do with USB command errors is normally due to bad drivers for the switch, bad cable, or bad drivers for your USB port, so you could always try a different port/hub/cable.