The mod by Fix94 extract the IOS from the NAND, you don't need to do it manually first. But in my opinion, it's a good idea to extract the IOS manually and keep a copy, they are not available on NUS and might still be required in the future. (or keep a NAND Dump) Wiiflow works fine as long as your HDD is compatible. You might need a Y cable to provide enough power, or use an external powered USB device. Most users reported that only IOS with base58 was working, but it could be only that some WiiU doesn't like the other bases for USB (I had this problem once with a Wii, only base58 worked). I think some users reported installing base56 and it worked fine. So, install all three available bases and test them.