Speedest Installation macOS Sierra from Thunderbolt, MacBook Air
You can do a clean install easily:


Attach the USB installer 
Reboot holding the Option key
Hold on until you see the "install Mac OS Sierra" volume
Choose "Disk utility" in the Mac OS utilities screen
Select you HD and erase it
Format it in hfs+, usually with the name "Macintosh HD"
And finally choose "Install Mac OS"


You may want to backup your data first.