Here is my method of producing chlorine dioxide solution:
Take a small Kilner jar
and add 350ml of [tap] water.
In a shot glass that is large enough to be stable in the jar without the water overtopping put:
9.3ml of MMS (which is a 22.4% solution of sodium chlorite [NaClO₂], typically made as a 28% concentration of 80% pure NaClO₂).
2ml of concentrated (36%) hydrochloric acid (HCl). You can instead use 17ml of 5% or 21ml of 4% if you don't have the concentrate.
The reaction will start immediately, turning brown and emitting the chlorine dioxide (ClO₂) gas; close the lid of the jar and place it in a cupboard or dark place.
In the morning, the water and the reagents should be the same, light yellow colour; place the jar in the fridge for an hour.
After the jar and contents have chilled, pour the contents of the shot glass into another container for other uses (as a surface cleaner, etc).
Decant the chlorine dioxide solution (CDS) into coloured, labelled glass bottles and placed in the fridge.