The environment where you are doing the meditation should be properly cleaned. While cleaning, you should cultivate the motivation that since you are engaged in the task of accumulating great stores of merit by inviting the hosts of buddhas and bodhisattvas to this environment, it is important to have a clean place. You should see that all external dirt and dust around you is basically a manifestation of the faults and stains within your own mind. You should see that the most important aim is to purge these stains and faults from within your mind. Therefore, as you cleanse the environment, think that you are also purifying your mind. Develop the very strong thought that by cleaning this place you are inviting the hosts of buddhas and bodhisattvas who are the most supreme merit field, and that you will subsequently engage in a path that will enable you to purge your mind of the stains and delusions.