Firstly do not sanitize or use any chemicals for 3 days 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. (please note you do not need to use spa shock if using BioGuard)