The added warning is awesome. Do you have any remedies in case the user does make the mistake of creating watery nail polish?

This is nit-picky, but "usable" means liquified? Thinned?

You only have one tool which increases simplicity!

However, you may want to include a rag and/or remind users to have a clean workspace before beginning this guide.

Consider using bullet points to separate each individual action. (This is explained further in a comment for step 2).

Your steps are great! It might be faster to navigate everything if you put distinct actions separated as individual bullet points.

The following list shows what I mean by "distinct actions."

Squirt 2 drops of nail polish thinner into the dry nail polish.

Screw the top back onto the nail polish.

Shake the nail polish bottle.

Instead of using the ‘/‘ character, increase user-friendliness by using a conjunction like " thick and dry."