What he means is to take 3% hydrogen peroxide (H2O2) - it says 3% on the bottle which mean the concentration is .03 Moles H2O2 per 1 Liter of water. That part you dont needa know it sjust an explination of 3%... The peroxide comes as 3% so u dont needa do anything to it....

Next you gotta dilute it in 10 parts of water which just means measure out an amount of water (well say 10 mL) and then add 1 mL (one tenth)of hydrogen peroxide.

The 2nd part is just saying to add 15 mL of hydrogen peroxide straight from the bottle into 1 liter of water. and Vwalah... you got yourself a bath.

Good luck trying it out