I In that domain, French is not astrained to explicit rules, as German is, nor to a single rule, and the choice of the voice in directives of a technical nature is yours but there are trends. Nevertheles, you might have to conform to the policy of the particular firm you work for.
The trend is to use the active voice, the imperative mood and infinitival ...
As Reyedy's commented, there is no real rule outside author's preference.
You can write:
Mettez le bouton Marche-Arrêt sur la position Arrêt. (imperative)
Mettre le bouton Marche-Arrêt sur la position Arrêt. (active voice)
Le bouton Marche-Arrêt doit être sur la position Arrêt. (passive voice)
Once you select to use either 1. or 2., you should stick to ...
Not a native speaker of French. In my scientific field (mechanics, mathematics) usage of passive voice is made to a certain degree but not in the same amount as in English. Bear in mind that there are in French other passive structures (with the personal pronoun on or with a pronominal verb).
When imperative is used in English, this can by conveyed by either ...
In this case "I know not how it was" is saying that the author or teller of the story is saying that he can't explain it, but somehow, looking at this house, there was a pervasive sense of insufferable gloom. The "I know not how it was" is just that " he doesn't know how to explain it".