Is it better to use "se" or "ser + adjective" for passive voice?
 

If I wanted to say something as simple as

The ball was round.

Is it better to say

La pelota se redondea

or

La pelota fue redonda?

'The ball was round' is not a passive voice sentence.
You simply stated a characteristic of the ball. The structure is 'ser' + 'subjective complement (formerly called 'predicate adjective')':
La pelota era redonda.

Characteristics in the past take the imperfect tense in Spanish.

The passive voice, very common in English, is when the subject receives the action of the verb. The agent (the thing that caused the action) may be present, or it may be omitted. The structure is 'ser' + 'past participle':
The house was destroyed. (no agent)
The house was destroyed by the tornado. (agent)

The passive voice is formed the same way in both languages, which makes it convenient for the learner, but the passive voice is not used nearly as often in Spanish. The 'passive se' construct is preferred.

The house was destroyed.

Passive 'se':
Se destruyó la casa.

Passive voice:
La casa fue destruida.

I have taught the passive voice use and I have understood this.

The lady buy vegetables in the store.

I will became this sentence in active voice.

The vegetables was bought by the lady in the store.

If I'm wrong, please you feel you free to correcting me.

The lady buys vegetables in the store. (Active Voice)
The vegetables were bought. (Passive Voice, no agent)
The vegetables were bought by the lady in the store. (Passive Voice, with agent)

Hello Rusty.

Thank you for your reply.

I'm sorry for misunderstand the active voice time, and passive voice.

I got it your commentary.

For example.

The chocolates are buying in the store.
In this case the most important are the substantive who gives us the action.

Here in this other ones.

Juan bought the chocolate in the store.

In this case the important is the person the subject.
I would call this verbal time active voice.

And in the first one I'd call passive voice.

I wait your commentaries.

Some corrections for you.

What you want to say with Agent.

Thank you for remind me the word await.
And I understood all your advices.

Although you are agree with me that with only I say (I got it). That's the equivalent of I understood it.

You taught me that time before.

Yes, 'I got it' means 'I understood', but you can't say 'I got it your ...'. You revert back to the non-idiomatic 'understand', in that case - 'I understood your ...'.

Agent - the person who performed the action

The towels were washed. (no agent - we don't know who washed the towels)
The towels were washed by Lucy. (Lucy washed them. Lucy performed the action, so she is the agent.)

Thank you for your advice.

I understood everything you told me.

Sincerely yours.