Firstly, both ladder feet should sit firmly on the ground. Divide the ladder height by 4, then move the ladder base that far from the building. It is also advised that someone else supports the ladder whilst another uses it to reduce the risk of the ladder slipping or falling.
The HSE guidance is for employers to take simple, sensible precautions when using ladders and stepladders in the workplace. We recommend reading Safe use of ladders and stepladders: A brief guide, which complies with the Work at Height Regulations 2005 (WAHR).