Tutorials:
Websites:
Apps/Editing tools:
Grammarly Draft Autocrit After the Deadline Editing Minion Reverse Dictionary Quip
Other resources:
Dictionaries
Thesauruses
What I've found helpful: be aware of what mistakes are most common in your manuscript. For example, if you keep a tally in the first few chapters of what kind of errors you find and notice a lot of comma splices and subject-verb disagreements, when you're editing the next chapter, first edit it with the sole intention of making sure the subjects and verbs all agree. Then, read it again and keep an eye out for comma splices. Finally, go through it once more and fix any other mistakes. I've also noticed that often when I take a break from editing and come back to it later, I'm more likely to notice little errors that I might have missed earlier. Additionally, it never hurts to have another pair of eyes check your work!