Since you refer to a step by step guide already … Where do you start having problems with it?
And since /usr/bin/fastboot sounds like Linux … which Linux do you use?
And since /usr/bin/fastboot sounds like this fastboot was installed via means of the Linux you use (following the guide fastboot doesn’t end up there, I think) … can’t you just (depending on which Linux you use)
… or reinstall adb and fastboot via your Linux’s graphical software install center?