"Harry throws the ball"
1. Undergo-er becomes the subject of the sentence
"The ball..."
2. You define the verbs in the sentence and check in what tense they are written.
"throws" - present simple
3. You take to be and make sure it fits both the tense and the new subject of the sentence.
"The ball is..."
4. You take the main verb of the active sentence and put it in the past participle (voltooid deelwoord).
"The ball is thrown."
5. You decide whether it's necessary to put the old subject in a by... clause. This is only necessary when it's important that we know who did it/when this will provide important information.
"The ball is thrown by Harry."