Focus on the action: present passive
The process passive uses werden plus Partizip II. The thing affected becomes the subject, so werden agrees with it. The agent can be introduced with von plus dative.
Understand the idea
Process passive shifts attention from the person doing an action to the person or thing affected. The original accusative object commonly becomes the passive subject. The actor can be omitted when it is unknown, obvious or unimportant.
Build it step by step
Conjugate werden for the new subject and add the past participle. Singular subjects normally take wird; plural subjects take werden. An actor introduced with von is in dative. Other complements keep their own grammatical requirements; a dative object does not automatically turn into a nominative subject.
See it in a sentence
Der Vertrag wird heute unterschrieben.
Das Fahrrad wird gerade repariert.
A useful memory aid
Compare one active and one passive sentence while keeping the event the same. Notice which noun now controls the finite verb.
A common trap
Do not use sein when you mean an action in progress. Sein plus a participle often describes a resulting state, which is a different construction.