Find a clean workplace where you can easily clean up spills.
DO NOT USE ACETONE TO THIN THE NAIL POLISH. Acetone is a solvent for removing nail polish. It can make the nail polish work but, the end result is dull or lumpy nail polish.
You can use a few drops of 91% isopropyl alcohol or ethyl acetate as solvents for thinning nail polish.
Consider using bullet points to separate each individual action. (This is explained further in a comment for step 2).
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.
Open the bottle of dry nail polish setting the brush to the side.
Open the bottle of nail polish thinner.
Squirt 2 drops of nail polish thinner into the dry nail polish.
Screw the top back onto the dry nail polish and roll the bottle between your hands to mix. Shaking the bottle can put air bubbles in the nail polish and ruin a manicure.
Be careful to not not spill any nail polish thinner.
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.
If the nail polish is still dry or thick repeat steps 2-3 until the nail polish is finally thinned.
Do not apply too much nail polish thinner, this will result in watery nail polish.
Instead of using the '/' character, increase user-friendliness by using a conjunction like " thick and dry."
This is nit-picky, but "usable" means liquified? Thinned?
The added warning is awesome. Do you have any remedies in case the user does make the mistake of creating watery nail polish?