The passive voice can be expressed in two ways.
Personless passive
The so called oni-pasivo puts the focus on action, what happens to the object. Cf. man-passive found in many Germanic languages (but not for instance in English).
- Ŝi skribas la leteron → Oni skribas la leteron / La leteron oni skribas.
Here the information who performs the action is lost, but the focus is clearly on the action. This passive is easy to form: just replace the person pronoun with oni. Because of that easiness this is usually the most recommended way to express the passive voice.
Describing passive
The compound passive or esti-pasivo describes the change of state caused by the action.
- Ŝi skriba̲s la leteron → La letero estas skriba̲ta de ŝi.
As Eduardo stated use the same tense in the passive participle as you have in the corresponding active voice verb.
This passive has several points worth noticing:
- the focus is no longer in the action but in a description of the change of the state (this may sound a minor thing but in some contexts it may matter)
- what is conceptually an object (like the letter in your example), is now suddenly a subject (since you just describe the change); this may confuse people with some language backgrounds
- you can form this form only formfrom transitive verbs, i.e. you cannot say *estas kurata* (in comparison oni kuras is fine)
- in this kind of passive sentence de always marks the agent which causes problems should you need to use de for some other purpose (and de is the most overloaded preposition in Esperanto, see PIV)