It is very easy, just follow these tips.
ACTIVE: Gaudi designed La Sagrada Familia
PASSIVE: La Sagrada Familia was designed by Gaudi
1. Active subject becomes agent preceeded by the preposition by
2. Active object becomes passive subject
3. Auxiliary Be is used in the same tense as the main verb
4. Main verb always in past participle
This link will help you to understand all the changes in passive much better.
Once you have read the chart you can practice passives here, thanks to Caroline and Pearson Brown.