Fazemos uma repetição depois do alongamento.

In Portuguese, the subject pronoun is often omitted when the verb form already makes the subject clear.

• fazemos = we do / we make
• So (nós) fazemos is possible, but nós is usually unnecessary.

You would include nós only for emphasis, contrast, or clarity:

• Nós fazemos uma repetição, eles fazem duas.
  We do one repetition, they do two.

Fazemos is the present indicative of fazer.

In context, the Portuguese present tense can cover several ideas that English often separates:

• a habitual action: We do a repetition after stretching
• a routine/instructional action: We do one repetition after the stretch
• sometimes even a near-present procedural sense, depending on context

So the exact English translation depends on the situation, but the Portuguese form is simply present tense.

Yes, fazer is an irregular verb, but fazemos is the normal nós form.

A few present-tense forms are:

• eu faço
• você/ele/ela faz
• nós fazemos
• vocês/eles/elas fazem

So fazemos is the correct we form of fazer.

Yes, fazer usually means to do or to make, but it is also very common in exercise, routine, and activity contexts.

For example:

• fazer exercício = to exercise
• fazer uma pausa = to take a break
• fazer uma repetição = to do one repetition / one rep

So this use sounds natural in Portuguese.

Uma repetição literally means one repetition or a repetition.

Here:

• uma is the feminine singular indefinite article / number one
• repetição is a feminine noun

In exercise language, repetição is the standard word for a rep. So uma repetição can mean:

• one repetition
• one rep

If there were more, you could say:

• duas repetições
• dez repetições

Because Portuguese normally contracts de + o into do.

So:

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

That means:

• depois do alongamento = after the stretching

Using de o alongamento would sound incorrect in standard Portuguese.

Portuguese often uses definite articles more than English does.

So where English might say:

• after stretching

Portuguese naturally says:

• depois do alongamento
• literally, after the stretching

This is very common and natural. The article does not necessarily make it sound overly specific in the way English might.

Alongamento is a noun meaning stretching or a stretching exercise/session, depending on context.

It comes from the verb alongar = to stretch.

So:

• alongar = to stretch
• alongamento = stretching / stretch routine / stretch

In fitness or physical training contexts, alongamento is a very common word.

Yes, absolutely.

Both are natural:

• Fazemos uma repetição depois do alongamento.
• Depois do alongamento, fazemos uma repetição.

The difference is mostly emphasis:

• starting with Fazemos... focuses first on the action
• starting with Depois do alongamento... focuses first on the time/order

Portuguese word order is fairly flexible here.

Because depois normally combines with de before a noun or pronoun.

Examples:

• depois do treino = after training
• depois da aula = after class
• depois de você = after you

So in this sentence:

• depois de + o alongamento
• which contracts to depois do alongamento

That structure is standard Portuguese.

In standard Portuguese, repetição is the normal full word. In gym or fitness contexts, people may sometimes use shortened or borrowed language informally, but for learners, repetição is the safest and most natural choice.

Examples:

• faça 10 repetições
• vamos fazer mais uma repetição

So if you want correct, broadly useful Portuguese, use repetição.

The ending -ção is very common in Portuguese and is pronounced roughly like a nasal sown, but not exactly like English.

A helpful approximation is:

• re-pe-ti-SOWN
  with the last syllable nasal

A few pronunciation notes:

• ti here sounds like chee in many Brazilian accents, so repetição may sound roughly like re-pe-ti-SOWN or re-pe-chi-SOWN
• ão is nasal, which is important in Brazilian Portuguese

You do not need a perfect English equivalent; it is best learned by listening and repeating.