This duo can get jumbled up pretty easily in a manuscript, so here is a bit of an explanation to help you figure them out.
Think of the two words as physical versus speculative:
Farther is a physical that can be measured in real distance (think how FAR can you go): scooted inches, walked a mile down the trail etc. Or in terms of our word: scooted farther behind the wall, walked farther down the trail.
Further is speculative…more of an abstract time or advancement: thinking further on a subject before giving an opinion, discussing this further when we have more time.
Marianne slid farther back on the surfboard as she propelled it through the water. The bright sun and cool water inspired her to let her mind drift further into the future, picturing herself winning next weekend's tournament.
Add these two words to your self-edit checklist and assess each use to its appropriate relation...is it a physical distance, or just a speculative descriptive?