How do I tighten a loose kitchen faucet?
The tool you need to get behind the sink to tighten the retainer nut is called a basin wrench.  You can buy them anywhere for $8 to $40.  They swivel to fit and have extended handle so you can turn it from below the sink level.  Yes, you have to do it by sense of feel, or position a mirror to help you see the nut. Google sink wrench or basin wrench and you will see dozens of them.

Plumb Pak PP840-7 11 Inch Basin Wrench
$8 online