There are plenty of walkthroughs at GameFaqs. However, I wanted to offer you Nintendo's Official Player's Guide for Earthbound. This covers the entire game! And this PDF isn't a scan - this is an export from the source file! You can't get better quality than this.

EarthBound Official Players Guide

Earthbound Player's Guide (Official)

Size: 50.2 MB Download


Here's an export of the Player's Guide in JPG. These are in a smaller resolution than the PDF.