Firstly do not sanitize or use any chemicals for 3 hours to let your water settle. If you are due to change your water, dump your water and start fresh. (use a Pipe Degreaser before you dump water so pipes are cleaned out completely).
Now you can start using BioGuard Armour, make sure you follow the dosing instructions.
NOTE: You do not need to use spa shock if using BioGuard Armour sanitiser.